Currently this only handles top-level nominal types. We're just trying to
emulate what the debugger does when it needs to go from a mangled name to
an AST node, so it's okay that the cases handled here are very restricted.
We just want to make sure that the debugger is /able/ to do what it needs
to do.
This does not yet handle nested (non-top-level) values; that will require
changes to DeclContext::lookupQualified.
Part of rdar://problem/17632175
Swift SVN r21690
