Revert "[libc++] Refactoring __sync_* builtins; NFC (Reland)"

This reverts commit r307595. The commit had some issues that needed
to first be addressed in review.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@307746 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed