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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171773 91177308-0d34-0410-b5e6-96231b3b80d8