DebugInfo: Add MDLexicalBlockBase::getLine(), etc.

Add a few functions from `DILexicalBlock` to `MDLexicalBlockBase`,
leaving `DILexicalBlock` a simple wrapper.

IMO, the new functions (`getLine()` and `getColumn()`) don't really
belong in the base class, but to simplify transitioning old code it
seems like the right incremental step.  I've explicitly deleted them in
`MDLexicalBlockFile`, and eventually the callers should be updated to
downcast to `MDLexicalBlock` directly and the forwarding functions
removed.

llvm-svn: 234842