No functional changes intended.

Like #43008 (f6689991), but _much more aggressive_.

Replaced by adding extra imports, adding hidden code (`# ...`), modifying
examples to be runnable (sorry Homura), specifying non-Rust code, and
converting to should_panic, no_run, or compile_fail.

Remaining "```ignore"s received an explanation why they are being ignored.

Instead of requesting the region maps for the entire crate, request for
a given item etc. Several bits of code were modified to take
`&RegionMaps` as input (e.g., the `resolve_regions_and_report_errors()`
function). I am not totally happy with this setup -- I *think* I'd
rather have the region maps be part of typeck tables -- but at least the
`RegionMaps` works in a "parallel" way to `FreeRegionMap`, so it's not
too bad. Given that I expect a lot of this code to go away with NLL, I
didn't want to invest *too* much energy tweaking it.

In MIR construction, operands need to live exactly until they are used,
which is during the (sub)expression that made the call to `as_operand`.

Before this PR, operands lived until the end of the temporary scope,
which was sometimes unnecessarily longer and sometimes too short.

Fixes #38669.

This reduces the number of dynamic drops in libstd from 1141 to 899.
However, without this change, the next patch would have created much
more dynamic drops.

A basic merge unswitching hack reduced the number of dynamic drops to
644, with no effect on stack usage. I should be writing a more dedicated
drop unswitching pass.

No performance measurements.

Constructs a TerminatorKind::SwitchInt for an equivalent conditional true-false branch.

This ends up not really mattering because we generate a plain conditional branch in LLVM either
way.

Also use a Cow to avoid full Vec for all SwitchInts

This speeds up compilation by 3--6% across most of rustc-benchmarks.

Use it instead of a `panic` for inexhaustive matches and correct the
comment. I think we trust our match-generation algorithm enough to
generate these blocks, and not generating an `unreachable` means that
LLVM won't optimize `match void() {}` to an `unreachable`.

non-MIR translation is still not supported for these and will happily ICE.

This is a [breaking-change] for many uses of slice_patterns.

clarify comments and panic message