LexicalScopes: Use debug info hierarchy pervasively

Pervasively use the types provided by the debug info hierarchy rather
than `MDNode` in `LexicalScopes`.

I noticed (again, I guess, based on comments in the implementation?)
that `DILexicalBlockFile::getScope()` returns something different from
`DILexicalBlockFile::getContext()`.  I created a local helper for
getting the same logic from `MDLexicalBlockFile` called
`getScopeOfScope()`.  I still don't really understand it, but I've added
some FIXMEs and I'll come back to it (I suspect the way we encode these
objects isn't really ideal).

Note that my previous commit r233610 accidentally changed behaviour in
`findLexicalScope()` -- it transitioned from a call to
`DILexicalBlockFile::getScope()` to `MDLexicalBlockFile::getScope()`
(sounds right, doesn't it?) -- so I've fixed that as a drive-by.  No
tests failed with my error, so it looks like we're missing some coverage
here... when I come back to understand the logic, I'll see if I can add
some.

Other than the fix to `findLexicalScope()`, no functionality change.

llvm-svn: 233640