Tracking Issue for `guard` macro fragment specifier

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This is a tracking issue for the feature "guard macro fragment specifier" accepted in #152832 (comment).
The feature gate for the issue is #![feature(macro_guard_matcher)].

Introduce a new macro fragment specifier guard to match MatchArmGuard, to make it easier for macro authors to match the recently stabilized if_let_guard feature (#51114).

This resolves #152313, because we either introduce a new :guard matcher or must refactor matches! and assert_matches! into built-in macros. Considering that more users will benefit from :guard, we choose to do so.

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Steps

• Implement the :guard fragment specifier for macro_rules and decl_macro
  Add macro matcher for guard fragment specifier #153702
• Use :guard in mathces! and assert_matches! to support if-let guards
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• (fmease writing) Should { really be in the follow set? I'm not quite sure it should.
  • The condition of match guards can be an "unrestricted" expressions precisely because the guard can only be followed by => contrary to the conditions of if & while expressions and the iterator of for loops which are expressions without struct expressions.
  • So while if S {} successfully parses as an expression (namely as if (S) {}), macro matcher $g:guard {} currently rejects if S {} as input because it parses if (S {}) and looks for extra { } which aren't there of course.

Implementation history

• Support if-let-guard in matches! and assert_matches! #152832
  The initial attempt used :tt to match if-let guards, but :tt greedily captured the trailing comma and the assertion message. So we decide to introduce a new :guard matcher.
• Add macro matcher for guard fragment specifier #153702
• Make the expansion of guard metavars begin guard non-terminals #155182