Debug Info: improve the verifier to check field types.

Make sure the context and type fields are MDNodes. We will generate
verification errors if those fields are non-empty strings.
Fix testing cases to make them pass the verifier.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187106 91177308-0d34-0410-b5e6-96231b3b80d8