Merge ArtificialLocation into ApplyDebugLocation and make a clear

Merge ArtificialLocation into ApplyDebugLocation and make a clear
distinction between the different use-cases. With the previous default
behavior we would occasionally emit empty debug locations in situations
where they actually were strictly required (= on invoke insns).
We now have a choice between defaulting to an empty location or an
artificial location.

Specifically, this fixes a bug caused by a missing debug location when
emitting C++ EH cleanup blocks from within an artificial function, such as
an ObjC destroy helper function.

rdar://problem/19670595

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@228003 91177308-0d34-0410-b5e6-96231b3b80d8