Revert "[clang] adds unary type transformations as compiler built-ins"

This reverts commit bc60cf23.
Doesn't build on Windows and breaks gcc 9 build, see
https://reviews.llvm.org/D116203#3722094 and
https://reviews.llvm.org/D116203#3722128

Also revert two follow-ups. One fixed a warning added in
bc60cf23, the other
makes use of the feature added in bc60cf23
in libc++:

Revert "[libcxx][NFC] utilises compiler builtins for unary transform type-traits"
This reverts commit 06a1d917.

Revert "[Sema] Fix a warning"
This reverts commit c85abbe8.