[libc++] Remove C++11 work-arounds in src.

Building libcxx requires at least C++17 so remove the old work-arounds.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D96200

GitOrigin-RevId: 2544060e718098c2cf736e2667ee007fbc58da50
2 files changed