Add C++ headers <barrier> <concepts> <execution> <latch> <numbers> <semaphore>

I missed adding these to the libc++ Makefile, when importing
llvm-project 11.0.0-rc1, even though they were supplied by upstream.

While here, update OptionalObsoleteFiles.inc to add these new headers,
and cleanup old cruft.

Reported by: yuri
Submitted by: jkim (Makefile diff)
PR: 255374
MFC after: 3 days

(cherry picked from commit 95aa617e4bf09fcc813b1bab3d0dbf4b606807b1)