MFC r246487 (by theraven):

  Import new libc++ to head.  Various small fixes and cleanups.

MFC r249998:

  Merge libc++ trunk r180598.  Contains several minor cleanups and bug
  fixes, no major changes.
MFC r250392:

For some reason, the gcc intrinsics header tmmintrin.h was imported with
two copies of itself pasted together.  Remove the extraneous copy.