The wasm targets don't support gdb anyway so there's no need for this
section there.

In most places, we use a span when emitting `expected...found` errors.
However, there were a couple of places where we didn't use any span,
resulting in hard-to-interpret error messages.

This commit attaches the relevant span to these notes, and additionally
switches over to using `note_expected_found` instead of manually
formatting the message

When refering to or suggesting raw identifiers, refer to them with `r#`.

Fix #65634.

Currently, rustfix has no notion of mutually exclusive suggestions. When
it processes issue-59756, it will attempt to apply two mutually
exclusive suggestions for the same span, causing an error.

Fixes #66910

We have several different kinds of suggestions we can try to make when
type coercion fails. However, we were previously only emitting these
suggestions from `demand_coerce_diag`. This resulted in the compiler
failing to emit applicable suggestions in several different cases, such
as when the implicit return value of a function had the wrong type.

This commit adds a new `emit_coerce_suggestions` method, which tries to
emit a number of related suggestions. This method is called from both
`demand_coerce_diag` and `CoerceMany::coerce_inner`, which covers a much
wider range of cases than before.

We now suggest using `.await` in more cases where it is applicable,
among other improvements.

Fixes #66787

Fixes #66702

LLVM 7 is over a year old, which should be plenty for compatibility. The
last LLVM 6 holdout was llvm-emscripten, which went away in #65501.

I've also included a fix for LLVM 8 lacking `MemorySanitizerOptions`,
which was broken by #66522.