[clang] [clang] Implement -Wmissing-include-dirs (PR #94827)

[via cfe-commits]

https://github.com/zmodem commented:

Thanks! The main missing thing is a test for this. Have a look in `clang/test/Driver/` to see what the tests can look like.

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Some existing tests are failing, see https://buildkite.com/llvm-project/github-pull-requests/builds/70953#018ff782-9ba1-426b-bb3e-7c0e93533ba4

For example it seems clang/test/Frontend/warning-poison-system-directories.c is failing because it builds with -Werror and passes -I flags with directories that don't exist on Windows.

clang/test/Driver/cl-pch.c also failed, but it's not clear to me why.

Maybe if the flag is not enabled by default, these test failures will go away.

https://github.com/llvm/llvm-project/pull/94827