Change `LLParser` to require a non-null `scope:` field for both
`MDLocation` and `MDLocalVariable`. There's no need to wait for the
verifier for this check. This also allows their `::getImpl()` methods
to assert that the incoming scope is non-null.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233394 91177308-0d34-0410-b5e6-96231b3b80d8
