Use PathBuf instead of String where applicable

r? @jseyfried

Point at whole method call instead of args

To avoid confusion in cases where the code is

```rust
fn foo() {}
/ foo(
|     bar()
|     ^^^ current diagnostics point here for arg count mismatch
| );
|_^ new diagnostic span points here
```

as this leads to confusion making people think that the diagnostic is
talking about `bar`'s arg count, not `foo`'s.

Point at `fn`s definition on arg mismatch, just like we do for closures.

Re #42855, Fix #45633.

Use spans for -Z external-macro-backtrace

```
% rustc ui/type-check/cannot_infer_local_or_vec.rs -Z external-macro-backtrace
error[E0282]: type annotations needed
  --> <vec macros>:3:1
   |
1  | / ( $ elem : expr ; $ n : expr ) => (
2  | | $ crate :: vec :: from_elem ( $ elem , $ n ) ) ; ( $ ( $ x : expr ) , * ) => (
3  | | < [ _ ] > :: into_vec ( box [ $ ( $ x ) , * ] ) ) ; ( $ ( $ x : expr , ) * )
   | | ^^^^^^^^^^^^^^^^^^^^^
   | | |
   | | cannot infer type for `T`
4  | | => ( vec ! [ $ ( $ x ) , * ] )
   | |______________________________- in this expansion of `vec!`
   |
  ::: ui/type-check/cannot_infer_local_or_vec.rs
   |
12 |       let x = vec![];
   |           -   ------ in this macro invocation
   |           |
   |           consider giving `x` a type

error: aborting due to previous error
```

make MIR type checker handle a number of other cases

The existing type checker was primarily used to verify types, but was skipping over a number of details. For example, it was not checking that the predicates on functions were satisfied and so forth. This meant that the NLL region checker was not getting a lot of the constraints it needed. This PR closes those gaps. It also includes a bit of refactoring for the way that we store region values, encapsulating the bit matrix over into its own module and improving the data structures in use.

This is mostly work by @spastorino being ported over from nll-master.

r? @arielb1 or @pnkfelix

Fix visible_parent_map to choose globally minimal paths

Fix #46112: visible_parent_map construction needs a BFS over whole crate forest to get globally minimal paths.

(There are other latent bugs that were e.g. causing this test case to have weirdness like `<unnamed>` in the diagnostic output. Those bugs are not fixed here, since they are issues long-standing in the stable channel.)

Download crosstool-ng from GitHub

Workaround the current problem where http://crosstool-ng.org was done, causing all non-x86 jobs to fail spuriously (cc #40474).

If http://crosstool-ng.org becomes online before this PR is merged, this PR should be closed and the tree should be reopened.

Temporary workaround since crosstool-ng.org was down. Consider mirroring
the release tarball as a more permanent solution.

Plus an extra assertion.

Converting a `RegionElementIndex` to a `Location` is O(n) though could
trivially be O(log n), but we don't do it that much anyhow -- just on
error and debugging.

These tests had FIXMEs for errors that were not previously being
reported.

macros: hygienize use of `core`/`std` in builtin macros

Today, if a builtin macro wants to access an item from `core` or `std` (depending `#![no_std]`), it generates `::core::path::to::item` or `::std::path::to::item` respectively (c.f. `fn std_path()` in `libsyntax/ext/base.rs`).

This PR refactors the builtin macros to instead always emit `$crate::path::to::item` here. That is, the def site of builtin macros is taken to be in `extern crate core;` or `extern crate std;`. Since builtin macros are macros 1.0 (i.e. mostly unhygienic), changing the def site can only effect the resolution of `$crate`.

r? @nrc

Otherwise, `run-pass/typeck-fn-to-unsafe-fn-ptr.rs` fails the MIR type checker.

Record all imports (`use`, `extern crate`) in the crate metadata

This PR adds non-`pub` `use` and `extern crate` imports in the crate metadata since hygienic macros invoked in other crates may use them. We already include all other non-`pub` items in the crate metadata. This improves import suggestions in some cases.

Fixes #42337.

r? @nrc

Implement impl Trait lifetime elision

Fixes #43396.

There's one weird ICE in the interaction with argument-position `impl Trait`. I'm still debugging it-- I've left a test for it commented out with a FIXME.

Also included a FIXME to ensure that `impl Trait` traits are caught under the lint in https://github.com/rust-lang/rust/issues/45992.

r? @nikomatsakis

Resolve absolute paths as extern under a feature flag

cc https://github.com/rust-lang/rust/issues/44660
r? @nikomatsakis