`//@ needs-matching-clang` is misleading

[Zalathar]

Via #126155, I discovered that the //@ needs-matching-clang compiletest header is a lot more strict than one might assume from its name.

Tests with this header are not run by default, regardless of whether a “matching clang” is available or not. Instead, they only run if environment variable RUSTBUILD_FORCE_CLANG_BASED_TESTS is set.

The only test job that sets this variable is x86_64-gnu-debug, but that job currently doesn't actually run any tests! (Though that might change as a result of #126155.)

@rustbot label +A-compiletest +A-testsuite

[Zalathar]

For example, this might have caused the test added by #116264 to never actually run in CI.

[Zalathar]

Looks like there are a few tests that are broken, as a result of not actually being run in CI:

#126155 (comment)