swift234

Drop the separate "LookupName" type for discriminated lookup.

Now that there's just one entry point for discriminated lookup, there's not
really a need for this extra abstraction.

One thing still up in the air is unqualified lookup support for discriminator
preferences (for, say, breakpoint conditions), but we'll cross that bridge
when we come to it.

Part of rdar://problem/17632175

Swift SVN r21755