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

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.

Approved by:	so
Security:	EN-21:18.libc++
Reported by:	yuri
Submitted by:	jkim (Makefile diff)
PR:		255374
MFC after:	3 days

(cherry picked from commit 95aa617e4bf09fcc813b1bab3d0dbf4b606807b1)
(cherry picked from commit 70e13c4cffd5ff7a70296bc5c4c3b7525c278b1d)