swift234

Implement <rdar://22207630> QoI: UncurriedCandidate should handle ClosureExprs

This standardizes processing of callees in invalid applyexprs, eliminating
bogus diagnostics like:
t.swift:6:2: error: cannot invoke closure of type '() -> _' with an argument list of type '()'

we now properly diagnose the example in closure/closures.swift as ambiguous,
but don't do a particularly good job of saying why.  That is to follow.