[clang] [clang-tools-extra] [libcxx] [lldb] [llvm] Add clang basic module directory (PR #93388)

[Louis Dionne via llvm-commits]

ldionne wrote:

For tidying up the libc++ review queue, please remove the `libc++` tag once you rebase this (the small changes in `libcxx` will go away).

https://github.com/llvm/llvm-project/pull/93388