[clang] [clang] Add support for SerenityOS (PR #187941)

[Lucas Chollet via cfe-commits]

LucasChollet wrote:

> If /usr/local stays in the search path, at least add a test that exercises it, but clang/test/Driver/Inputs/serenity_tree/usr/local/.keep is not needed.

I already removed it in 02d03ead, simply forgot to remove `clang/test/Driver/Inputs/serenity_tree/usr/local/.keep`. This is now done (b6a0b1d).

https://github.com/llvm/llvm-project/pull/187941