[clang] clang: Fix tests when built with DEFAULT_SYSROOT (PR #229417)

[Sam Elliott via cfe-commits]

lenary wrote:

> Unrelated to this, but we really need to improve clang configuration files. I've been thinking about making an RFC on the subject, because what we have is very limited.

I know you're not the only one to say so. I think we have some different pressures here, but various people with opinions will be at the Embedded Toolchains workshop at LLVM Dev, so that might be a good place to discuss anything you have, if you've managed to write an RFC before that.

https://github.com/llvm/llvm-project/pull/229417