Prevent DILocation::getMergedLocation() from creating invalid metadata.

The function's new implementation from r340583 had a bug in it that
could cause an invalid scope to be generated when merging two
DILocations with no common ancestor scope.

This patch detects this situation and picks the scope of the first
location. This is not perfect, because the scope is misleading, but on
the other hand, this will be a line 0 location.

rdar://problem/43687474

Differential Revision: https://reviews.llvm.org/D51238

llvm-svn: 340672