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

[Rodrigo Salazar via llvm-commits]

4-rodrigo-salazar wrote:

Seeing unrelated test timeouts on some of the build checks. Will merge up against main tomorrow to see if it looks better.

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