Verifier: Move checks over from DIDescriptor::Verify()

Move over some more checks from `DIDescriptor::Verify()`, and change
`LLParser` to require non-null `file:` fields in compile units.

I've ignored the comment in test/Assembler/metadata-null-operands.ll
since I disagree with it.  At the time that test was written (r229960),
the debug info verifier wasn't on by default, so my comment there is in
the context of not expecting the verifier to be useful.  It is now, and
besides that, since r233394 we can check when parsing textual IR whether
an operand is null that shouldn't be.

llvm-svn: 233654