swift234

Change qualified lookup to take a (DeclName, discriminator) pair.

This is in preparation for allowing demangling to match up with a particular
AST node, even if there are two private decls with the same name in the same
module. This commit doesn't actually change anything about the lookup; the
next commits will start adding support for the various lookups-with-
discriminators. (This is mostly only useful for the debugger.)

Part of rdar://problem/17632175

Swift SVN r21689