Make the expansion of guard metavars begin guard non-terminals

[fmease]

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

#![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.

[rustbot]

r? @petrochenkov

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[petrochenkov]

@bors r+

[rust-bors]

📌 Commit fc2c72c has been approved by petrochenkov

It is now in the queue for this repository.