[compiler-rt] [compiler-rt] Fix detection of warnings flags (PR #125602)

[Eli Schwartz via llvm-commits]

https://github.com/eli-schwartz requested changes to this pull request.

> It appears that these checks report true without -Werror since they only
> trigger a -Wunknown-warning-option that does not fail the check.

Hold on,

I modified the test to use ECHO_OUTPUT_VARIABLE so as to get actual logging of what is happening. This is with clang.

```
-- Performing Test COMPILER_RT_HAS_WBUILTIN_DECLARATION_MISMATCH_FLAG
warning: unknown warning option '-Wbuiltin-declaration-mismatch'; did you mean '-Wmissing-declarations'? [-Wunknown-warning-option]
1 warning generated.
-- Performing Test COMPILER_RT_HAS_WBUILTIN_DECLARATION_MISMATCH_FLAG - Failed
```

The check is failing as it should, because:
https://github.com/llvm/llvm-project/blob/d810c741ad7a5f5bee8fe833b9ee4023446dd39c/compiler-rt/cmake/Modules/BuiltinTests.cmake#L90C3-L99

This yanks out some regex patterns from cmake internals and does string matching, one of which is:
```
    FAIL_REGEX "unknown .*option"                          # Clang
```

that does manage to successfully match here.

So it's not clear to me under what circumstances this check should ever detect success in COMPILER_RT_HAS_WBUILTIN_DECLARATION_MISMATCH_FLAG. I can't manage it myself, at least.

https://github.com/llvm/llvm-project/pull/125602