Verifier: Check fields of MDVariable subclasses

Check fields from `MDLocalVariable` and `MDGlobalVariable` and change
the accessors to downcast to the right types.  `getType()` still returns
`Metadata*` since it could be an `MDString`-based reference.

Since local variables require non-null scopes, I also updated `LLParser`
to require a `scope:` field.

A number of testcases had grown bitrot and started failing with this
patch; I committed them separately in r233349.  If I just broke your
out-of-tree testcases, you're probably hitting similar problems (so have
a look there).

llvm-svn: 233389