The implementation itself only requires changes to trans, but
a few additional bugs concerning the handling of fat pointers
had to be fixed.

const eval errors outside of true constant enviroments are not reported anymore, but instead forwarded to a lint.

paths, and construct paths for all definitions. Also, stop rewriting
DefIds for closures, and instead just load the closure data from
the original def-id, which may be in another crate.

of xxx_local.

they are being used as an opaque "position identifier"

A DST value and a fat pointer to it have the same representation, all we
have to do is to adjust the type of the datum holding the pointer.

That allows us to keep using trans_into() in case of adjustments that
may actually be ignored in trans because they are a plain deref/ref pair
with no overloaded deref or unsizing.

Unoptimized(!) benchmarks from servo/servo#7638

Before
```
test goser::bench_clone                          ... bench:      17,701 ns/iter (+/- 58) = 30 MB/s
test goser::bincode::bench_decoder               ... bench:      33,715 ns/iter (+/- 300) = 11 MB/s
test goser::bincode::bench_deserialize           ... bench:      36,804 ns/iter (+/- 329) = 9 MB/s
test goser::bincode::bench_encoder               ... bench:      34,695 ns/iter (+/- 149) = 11 MB/s
test goser::bincode::bench_populate              ... bench:      18,879 ns/iter (+/- 88)
test goser::bincode::bench_serialize             ... bench:      31,668 ns/iter (+/- 156) = 11 MB/s
test goser::capnp::bench_deserialize             ... bench:       2,049 ns/iter (+/- 87) = 218 MB/s
test goser::capnp::bench_deserialize_packed      ... bench:      10,707 ns/iter (+/- 258) = 31 MB/s
test goser::capnp::bench_populate                ... bench:         635 ns/iter (+/- 5)
test goser::capnp::bench_serialize               ... bench:      35,657 ns/iter (+/- 155) = 12 MB/s
test goser::capnp::bench_serialize_packed        ... bench:      37,881 ns/iter (+/- 146) = 8 MB/s
test goser::msgpack::bench_decoder               ... bench:      50,634 ns/iter (+/- 307) = 5 MB/s
test goser::msgpack::bench_encoder               ... bench:      25,738 ns/iter (+/- 90) = 11 MB/s
test goser::msgpack::bench_populate              ... bench:      18,900 ns/iter (+/- 138)
test goser::protobuf::bench_decoder              ... bench:       2,791 ns/iter (+/- 29) = 102 MB/s
test goser::protobuf::bench_encoder              ... bench:      75,414 ns/iter (+/- 358) = 3 MB/s
test goser::protobuf::bench_populate             ... bench:      19,248 ns/iter (+/- 92)
test goser::rustc_serialize_json::bench_decoder  ... bench:     109,999 ns/iter (+/- 797) = 5 MB/s
test goser::rustc_serialize_json::bench_encoder  ... bench:      58,777 ns/iter (+/- 418) = 10 MB/s
test goser::rustc_serialize_json::bench_populate ... bench:      18,887 ns/iter (+/- 76)
test goser::serde_json::bench_deserializer       ... bench:     104,803 ns/iter (+/- 770) = 5 MB/s
test goser::serde_json::bench_populate           ... bench:      18,890 ns/iter (+/- 69)
test goser::serde_json::bench_serializer         ... bench:      75,046 ns/iter (+/- 435) = 8 MB/s
```

After
```
test goser::bench_clone                          ... bench:      16,052 ns/iter (+/- 188) = 34 MB/s
test goser::bincode::bench_decoder               ... bench:      31,194 ns/iter (+/- 941) = 12 MB/s
test goser::bincode::bench_deserialize           ... bench:      33,934 ns/iter (+/- 352) = 10 MB/s
test goser::bincode::bench_encoder               ... bench:      30,737 ns/iter (+/- 1,969) = 13 MB/s
test goser::bincode::bench_populate              ... bench:      17,234 ns/iter (+/- 176)
test goser::bincode::bench_serialize             ... bench:      28,269 ns/iter (+/- 452) = 12 MB/s
test goser::capnp::bench_deserialize             ... bench:       2,019 ns/iter (+/- 85) = 221 MB/s
test goser::capnp::bench_deserialize_packed      ... bench:      10,662 ns/iter (+/- 527) = 31 MB/s
test goser::capnp::bench_populate                ... bench:         607 ns/iter (+/- 2)
test goser::capnp::bench_serialize               ... bench:      30,488 ns/iter (+/- 219) = 14 MB/s
test goser::capnp::bench_serialize_packed        ... bench:      33,731 ns/iter (+/- 201) = 9 MB/s
test goser::msgpack::bench_decoder               ... bench:      46,921 ns/iter (+/- 461) = 6 MB/s
test goser::msgpack::bench_encoder               ... bench:      22,315 ns/iter (+/- 96) = 12 MB/s
test goser::msgpack::bench_populate              ... bench:      17,268 ns/iter (+/- 73)
test goser::protobuf::bench_decoder              ... bench:       2,658 ns/iter (+/- 44) = 107 MB/s
test goser::protobuf::bench_encoder              ... bench:      71,024 ns/iter (+/- 359) = 4 MB/s
test goser::protobuf::bench_populate             ... bench:      17,704 ns/iter (+/- 104)
test goser::rustc_serialize_json::bench_decoder  ... bench:     107,867 ns/iter (+/- 759) = 5 MB/s
test goser::rustc_serialize_json::bench_encoder  ... bench:      52,327 ns/iter (+/- 479) = 11 MB/s
test goser::rustc_serialize_json::bench_populate ... bench:      17,262 ns/iter (+/- 68)
test goser::serde_json::bench_deserializer       ... bench:      99,156 ns/iter (+/- 657) = 6 MB/s
test goser::serde_json::bench_populate           ... bench:      17,264 ns/iter (+/- 77)
test goser::serde_json::bench_serializer         ... bench:      66,135 ns/iter (+/- 392) = 9 MB/s

```

This could be a [breaking-change] if your lint or syntax extension (is that even possible?) uses HIR attributes or literals.

Closes #26205.

Fixes #28109

auto_ref() handles fat pointers just fine and unlike ref_fat_ptr() does so
without creating an unnecessary copy of the pointer.

Combining them seemed like a good idea at the time, but turns out that
handling lifetimes separately makes it somewhat easier to handle cases
where we don't want the intrinsics, and let's you see more easily where
the start/end pairs are.

The issues that the comments referred to were fixed before the PR even
landed but we never got around to remove the hack of skipping the
lifetime start.

StructGEP seems clearer and probably does an even better job of the
micro-optimization that we have in GEPi.

The functions is useful for all kinds of fat pointers, but get_len()
just feels so wrong for trait object fat pointers. Let's use get_meta()
because that's rather neutral.

Currently `f32 % f32` will generate a link error on 32-bit MSVC because LLVM
will lower the operation to a call to the nonexistent function `fmodf`. Work
around in this in the backend by lowering to a call to `fmod` instead with
necessary extension/truncation between floats/doubles.

Closes #27859

- simd_eq, simd_ne, simd_lt, simd_le, simd_gt, simd_ge
- simd_shuffleNNN