[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.

llvm-svn: 202960