AsmParser: Reject invalid mismatch between forward ref and def

Don't assume that the forward referenced entity was of the same
global-kind as the new entity.

This fixes PR21779.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223754 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "AsmParser: Reject invalid mismatch between forward ref and def"

This reverts commit r223754.  I've upset the buildbots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223755 91177308-0d34-0410-b5e6-96231b3b80d8
Reland r223754

The commit is identical except a reference to `GV' should have been to
`GVal'.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223756 91177308-0d34-0410-b5e6-96231b3b80d8