[Layering] Move DebugLoc.h into the IR library. The implementation
already lived there and it is where it belongs -- this is the in-memory
debug location representation.

This is just cleanup -- Modules can actually cope with this, but that
doesn't make it right. After chatting with folks that have out-of-tree
stuff, going ahead and moving the rest of the headers seems preferable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202960 91177308-0d34-0410-b5e6-96231b3b80d8