Debug info: Refactor NoLocation and ArtificialLocation to use a common base
class and use it pervasively to restore debug locations.
Fixes an interaction between cleanup and EH that caused the location
to not be restored properly after emitting a landing pad.
rdar://problem/15208190

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199444 91177308-0d34-0410-b5e6-96231b3b80d8