swift234

Generalize the conditions in which we'll accept an ambiguous solution to
a constraint system in "allowFreeTypeVariables" mode.  Previously, we
only allowed a few specific constraints, now we allow any relational and
member constraints.  The later one is a big deal because it means that we
can allow ".Foo" expressions as ambiguous solutions, which CSDiags can
handle well.

This unblocks solving 23942743 and enables some minor improvements across
the board, including diagnosing things like this better:
  Optional(.none)  // now: generic parameter 'T' could not be inferred

That said, it also just permutes some non-awesome diagnostics.