[libcxx] [llvm] [libcxx] Provide locale conversions to tests through lit substitution (PR #86649)

[Louis Dionne via llvm-commits]

ldionne wrote:

FWIW I agree -- there are some things to change in this patch but I think the direction makes sense. It cleans up a lot of complexity in the test suite.

https://github.com/llvm/llvm-project/pull/86649