Add macro matcher for guard fragment specifier

[SpriteOvO]

Tracking issue #153104

This PR implements a new guard macro matcher to match if-let guards (specifically MatchArmGuard). In the upcoming PR, we can use this new matcher in the matches! and assert_matches! macros to support their use with if-let guards. (see #152313)

The original Expr used to represent a guard has been wrapped in a new Guard type, allowing us to carry the span information of the leading if keyword. However, it might be even better to include the if keyword in the Guard type as well? I've left a FIXME comment in the code.

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

r? @davidtwco

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[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    |
301 |     let guard_str = rewrite_guard(
    |                     ------------- arguments to this function are incorrect
302 |         context,
303 |         &arm.guard,
    |         ^^^^^^^^^^ expected `&Option<Box<Expr>>`, found `&Option<Box<Guard>>`
    |
    = note: expected reference `&std::option::Option<std::boxed::Box<rustc_ast::Expr>>`
               found reference `&std::option::Option<std::boxed::Box<rustc_ast::Guard>>`
note: function defined here
   --> src/tools/rustfmt/src/matches.rs:572:4
    |
572 | fn rewrite_guard(
    |    ^^^^^^^^^^^^^
573 |     context: &RewriteContext<'_>,
574 |     guard: &Option<Box<ast::Expr>>,
    |     ------------------------------

For more information about this error, try `rustc --explain E0308`.
[RUSTC-TIMING] rustfmt_nightly test:false 2.282
error: could not compile `rustfmt-nightly` (lib) due to 1 previous error