e.g. the following is now valid Torque code:

  macro TestA(implicit c: Context)() {}

  macro TestB(): bool {
    return TestA();
  }

This is handy for more flexible usage of generics that may or may not
use implicit parameters deep inside their specializations.

Note that this change doesn't change the fundamental rigor (or lack
thereof) around checking the usage of implicit parameters, which
already do not require '_' before their parameter identifier if
unused. It just silences errors in cases where a call site doesn't
implicitly pass a parameter that ultimately doesn't have a use site
and adds meaningful error messages in the case that it does.

Bug: v8:7793
Change-Id: I559d06c0864a7e79fe52bee5a9a7af9941889748
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2274127
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68618}