[CSBindings] Don't consider dependent member types even if they are wrapped in optionals

Because binding producer is going to attempt to unwrap optionals
and try the type, which would lead to infinite recursion because
dependent member types aren't bindable.

Resolves: rdar://problem/44770297

Author: xedin

lib/Sema/CSSolver.cpp

@@ -118,8 +118,11 @@ Optional<Type> ConstraintSystem::checkTypeOfBinding(TypeVariableType *typeVar,
     return None;
   }
 
-  // Don't bind to a dependent member type.
-  if (type->is<DependentMemberType>()) return None;
+  // Don't bind to a dependent member type, even if it's currently
+  // wrapped in any number of optionals, because binding producer
+  // might unwrap and try to attempt it directly later.
+  if (type->lookThroughAllOptionalTypes()->is<DependentMemberType>())
+    return None;
 
   // Okay, allow the binding (with the simplified type).
   return type;

validation-test/compiler_crashers_2_fixed/0174-rdar44770297.swift

@@ -0,0 +1,11 @@
+// RUN: not %target-swift-frontend -typecheck %s
+
+protocol P {
+  associatedtype A
+}
+
+func foo<T: P>(_: () throws -> T) -> T.A? {
+  fatalError()
+}
+
+_ = foo() { fatalError() } & nil