[libcxx] Stop using use c++ subdirectory for libc++ library

The new layout more closely matches the layout used by other compilers.
This is only used when LLVM_ENABLE_PER_TARGET_RUNTIME_DIR is enabled.

Differential Revision: https://reviews.llvm.org/D100869

GitOrigin-RevId: f749550cfe9f0bf2364abb2139835348587062ed
1 file changed