[libclang] When getting the cursor for an identifier inside a macro definition, check if

this was ever a macro name and return a specific CXCursor_MacroExpansion cursor in such a case,
instead of the generic CXCursor_MacroDefinition.

Checking for macro name makes sure the identifier is not part of the identifier list in a
function macro.
While, in general, resolving identifiers in macro definitions to other macros may not be completely accurate,
it greatly improves functionality such as give-me-the-definition-of-this, which was not working at all
inside macro definitions.

llvm-svn: 171773