New API for merging debug locations. NFC.

Given two debug locations the function getMergedLocation combines the
locations into a single location (which may be an empty location).
Please see https://reviews.llvm.org/D26256 for the discussion leading
up to this API.

Note the function is currently a stub.  This allows optimisations to
use the API although no location will actually be used.

This is patch 1 out of 8 for D26256.  As suggested by David Blaikie,
each change in D26256 has been broken out into a separate patch.

llvm-svn: 289661