[DebugInfo] Unify logic to merge DILocations. NFC.

This makes DILocation::getMergedLocation() do what its comment says it
does when merging locations for an Instruction: set the common inlineAt
scope. This simplifies Instruction::applyMergedLocation() a bit.

Testing: check-llvm, check-clang

Differential Revision: https://reviews.llvm.org/D39628

llvm-svn: 317524