IR: Fix MDType fields from unsigned to uint64_t

When trying to match the current schema with the new debug info
hierarchy, I downgraded `SizeInBits`, `AlignInBits` and `OffsetInBits`
to 32-bits (oops!).  Caught this while testing my upgrade script to move
the hierarchy into place.  Bump it back up to 64-bits and update tests.

llvm-svn: 229933