PR15067: Don't assert when a UCN appears in a C90 file.

Unfortunately, we can't accept the UCN as an extension because we're
required to treat it as two tokens for preprocessing purposes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173622 91177308-0d34-0410-b5e6-96231b3b80d8