Follow-up to r218292: Add more REVERTIBLE_TYPE_TRAITs.

r218292 reverted r197496 because it broke things. In addition to breaking
things, r197496 also made all traits starting with __is_ revertible.
Reinstantiate that part of r197496 because code out there (e.g. libc++) depends
on this behavior. Fixes PR21045.

llvm-svn: 218365