[DebugInfo] Handle same locations in DILocation::getMergedLocation

Revision 289661 introduced the function DILocation::getMergedLocation for
merging of debug locations. At the time is was simply a stub which always
returned no location. This patch modifies getMergedLocation to handle the
case where the two locations are the same or can't be discriminated.

Differential Revision: https://reviews.llvm.org/D28521

llvm-svn: 291809