Introduce SourceManager::getMacroArgExpandedLocation function.

If we pass it a source location that points inside a function macro argument,
the returned location will be the macro location in which the argument was expanded.
If a macro argument is used multiple times, the expanded location will
be at the first expansion of the argument.
 e.g.
   MY_MACRO(foo);
             ^
Passing a file location pointing at 'foo', will yield a macro location
where 'foo' was expanded into.

Make SourceManager::getLocation call getMacroArgExpandedLocation as well.

llvm-svn: 137794