This adds assembly and bitcode support for `MDLocation`. The assembly
side is rather big, since this is the first `MDNode` subclass (that
isn't `MDTuple`). Part of PR21433.
(If you're wondering where the mountains of testcase updates are, we
don't need them until I update `DILocation` and `DebugLoc` to actually
use this class.)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225830 91177308-0d34-0410-b5e6-96231b3b80d8
