AsmParser/Bitcode: Add support for MDLocation

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.)

llvm-svn: 225830