Reapply "DebugInfo: Loosen DILexicalBlock constructor"

This reverts commit r234225, reapplying r234222 in spirit.

This time, just include what the commit message actually describes:
loosen the `DILexicalBlock` constructor to require a
`MDLexicalBlockBase`, since that's what `DILexicalBlock` is wrapping.

llvm-svn: 234241