Use a more reliable method to determine whether

a FileID corresponds to a real file or to a
memory buffer.  The old method didn't work when
Clang was built Release, which meant it wasn't
a very good method at all.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236188 91177308-0d34-0410-b5e6-96231b3b80d8