[regression - rust2018]: unused_mut lint false positives on nightly

Fixes rust-lang#55344.

This commit filters out locals that have never been initialized for
consideration in the `unused_mut` lint.

This is intended to detect when the statement that would have
initialized the local was removed as unreachable code. In these cases,
we would not want to lint. This is the same behaviour as the AST borrow
checker.

This is achieved by taking advantage of an existing pass over the MIR
for the `unused_mut` lint and creating a set of those locals that were
never initialized.

r? @pnkfelix

[regression - rust2018]: unused_mut lint false positives on nightly

Fixes rust-lang#55344.

This commit filters out locals that have never been initialized for
consideration in the `unused_mut` lint.

This is intended to detect when the statement that would have
initialized the local was removed as unreachable code. In these cases,
we would not want to lint. This is the same behaviour as the AST borrow
checker.

This is achieved by taking advantage of an existing pass over the MIR
for the `unused_mut` lint and creating a set of those locals that were
never initialized.

r? @pnkfelix

Rollup of 17 pull requests

Successful merges:

 - #55576 (Clarify error message for -C opt-level)
 - #55633 (Support memcpy/memmove with differing src/dst alignment)
 - #55638 (Fix ICE in msg_span_from_free_region on ReEmpty)
 - #55659 (rustc: Delete grouping logic from the musl target)
 - #55719 (Sidestep link error from rustfix'ed code by using a *defined* static.)
 - #55736 (Elide anon lifetimes in conflicting impl note)
 - #55739 (Consume optimization fuel from the MIR inliner)
 - #55742 (Avoid panic when matching function call)
 - #55753 (borrow_set: remove a helper function and a clone it uses)
 - #55755 (Improve creation of 3 IndexVecs)
 - #55758 ([regression - rust2018]: unused_mut lint false positives on nightly)
 - #55760 (Remove intermediate font specs)
 - #55761 (mir: remove a hacky recursive helper function)
 - #55774 (wasm32-unknown-emscripten expects the rust_eh_personality symbol)
 - #55777 (Use `Lit` rather than `P<Lit>` in `ast::ExprKind`.)
 - #55783 (Deprecate mpsc channel selection)
 - #55788 (rustc: Request ansi colors if stderr isn't a tty)

Failed merges:

r? @ghost