[libcxx] [llvm] [libc++] Optionally support filecheck-based tests (PR #165769)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Drive by comment from a non libc++ contributor, so take my feedback as you will.

Why install a Python version of FileCheck when we could just install/build actual FileCheck? I'd prefer to avoid external dependencies whenever possible, especially when it's a much less battle tested reimplementation of something that we have in tree.

https://github.com/llvm/llvm-project/pull/165769