swift234

Remove an overzealous assert.

When we are able to type check an expression this assert is fine,
although of little value. In the context of failed type checks, though,
it can be actively harmful.

The issue is that we can attempt to solve smaller parts of the
constraint system and assign contextual types (e.g. for the arguments of
a function) despite not being able to drill into members.

Some of the diagnostics we emit in these cases are not terribly useful,
and I've opened rdar://problem/27982012 with one example.

Resolves:
  rdar://problem/25812474
  rdar://problem/26589865
  rdar://problem/27974638