directive as a macro expansion.

This is more of a "macro reference" than a macro expansion but it's close enough
for libclang's purposes. If it causes issues we can revisit and introduce a new
kind of cursor.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169666 91177308-0d34-0410-b5e6-96231b3b80d8