Rollup merge of #155182 - fmease:guard-exp-begins-guard, r=petrochenkov

Make the expansion of guard metavars begin guard non-terminals

While investigating something unrelated, I noticed a bug in the impl of unstable feature `macro_guard_matcher` (tracking issue: rust-lang/rust#153104). Namely, the following doesn't compile:

```rs
#![feature(macro_guard_matcher)]

macro_rules! a { ($guard:guard) => { b!($guard); }; }
macro_rules! b { ($guard:guard) => {}; }
a!(if true);
```

```
error: no rules expected `guard` metavariable
 --> src/lib.rs:3:41
  |
3 | macro_rules! a { ($guard:guard) => { b!($guard); }; }
  |                                         ^^^^^^ no rules expected this token in macro call
4 | macro_rules! b { ($guard:guard) => {}; }
  | -------------- when calling this macro
5 | a!(if true);
  | ----------- in this macro invocation
  |
note: while trying to match meta-variable `$guard:guard`
 --> src/lib.rs:4:19
  |
4 | macro_rules! b { ($guard:guard) => {}; }
  |                   ^^^^^^^^^^^^
  = note: this error originates in the macro `a` (in Nightly builds, run with -Z macro-backtrace for more info)
```

---

While I'm still skeptical of `guard` fragment specifiers in general (although I can't quite pinpoint why), I figured I should fix this issue.

Author: JonathanBrouwer