Don't diagnose failures to call symmetrically-typed binary operators

as a failure to convert the individual operand, since the operator
is likely conceptually generic in some way and the choice of any
specific overload is probably arbitrary.

Since we now fall back to a better-informed diagnostics point, take
advantage of this to generate a specialized diagnostic when trying to
compare values of function type with ===.

Fixes rdar://25666129.

This reverts commit 073f427942,
i.e. it reapplies 35ba809fd0 with a
test fix to expect an extra note in one place.

test/Constraints/patterns.swift

@@ -140,7 +140,7 @@ default: break
 
 // <rdar://problem/21995744> QoI: Binary operator '~=' cannot be applied to operands of type 'String' and 'String?'
 switch ("foo" as String?) {
-case "what": break // expected-error{{value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?}}
+case "what": break // expected-error{{expression pattern of type 'String' cannot match values of type 'String?'}}
 default: break
 }