Revert r197496, as it broke REVERTIBLE_TYPE_TRAITs from PCH files.

Also add a test to make sure that this doesn't break again. Fixes PR21036.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218292 91177308-0d34-0410-b5e6-96231b3b80d8