[Clang] Rewrite tests using subshells to set env variables

Now that we have the %readfile substitution, we can rewrite these tests
that were using env variable subshells to write the output of the
command into a file and then load it where it is needed using readfile.

This does involve one invocation of bash so that we are using the system
env binary, which does support redirection into a tool like grep. We
already do this in one LLVM test. I'm not happy about needing that, but
the more principled way to solve it involves reworking how in-process
builtins work within lit. That is something we want to do eventually,
but not something that I think should block this patch.

Reviewers: cmtice, petrhosek, ilovepi

Reviewed By: cmtice, ilovepi

Pull Request: https://github.com/llvm/llvm-project/pull/158446

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