swift234

Teach private-discriminator lookup to handle lookup into nominals.

This works using the nominal's normal lookupDirect to gather decls from the
original declaration and all its extensions, then filters them by module and
private-discriminator. This does not yet do the right thing for extensions
because we don't include the extension's module in a member's mangling except
in very specific circumstances. In order to make this work in general we'll
have to start doing this more generally.

Part of rdar://problem/17632175

Swift SVN r21785