[DebugInfo] getMergedLocation: Maintain the line number if they match

getMergedLocation returns a 'line 0' DILocaiton if the two locations
being merged don't perfecly match, even if they are in the same line but
a different column.

This commit adds support to keep the line number if it matches (but only
the column differs). The merged column number is the leftmost between the
two.

Reviewed By: dblaikie, orlando

Differential Revision: https://reviews.llvm.org/D135166