[libc++] Mark P2845R8 `__cpp_lib_format_path` and P2587R3 `__cpp_lib_to_string` as C++26#93255

ldionne merged 3 commits intollvm:mainllvm/llvm-project:mainfrom

StephanTLavavej:cxx26-feature-test-macrosStephanTLavavej/llvm-project:cxx26-feature-test-macrosCopy head branch name to clipboard