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

[Aiden Grossman via cfe-commits]

boomanaiden154 wrote:

> This seems to break some bots (running without lit's internal shell) since -u isn't a portable option on env:

That's pretty annoying. I guess it isn't required by any of the POSIX standards, but every single platform we have builbots for (even Solaris!) supports `env -u`, other then AIX.

I guess for now I can revert this.

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