[libc++] Remove support for Clang 9 and older.

I encountered this while reviewing an unrelated patch. Will land after
the CI passes.

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D114673

GitOrigin-RevId: 730dccb98622a82d1cad65e784ce5a8e8323e8cb
1 file changed