[clang] [clang] Rewrite tests relying on shell environment variable features (PR #156904)

[Paul Kirth via cfe-commits]

ilovepi wrote:

> Seems worth investigating adding support for `unset` in `lit`'s internal shell given how much repetition it would save here, TBH.

IIRC in the initial work last year it was deliberately decided not to support `unset`, since ists somewhat complex w/ its interactions, and you can have similar behavior w/ `env -u` for the specific tests that need it.

https://github.com/llvm/llvm-project/pull/156904