[Type checker] Handle literals based on superclass constraints.

When we form a literal whose type is a type parameter where the literal
conformance comes from a superclass constraint of that type parameter,
make sure we can property extract the type witness we need.

Fixes SR-7551 / rdar://problem/39860617.

(cherry picked from commit 8dbdf7f)

Author: DougGregor

lib/AST/ProtocolConformance.cpp

@@ -134,12 +134,6 @@ ProtocolConformanceRef::getTypeWitnessByName(Type type,
                                              ProtocolConformanceRef conformance,
                                              Identifier name,
                                              LazyResolver *resolver) {
-  // For an archetype, retrieve the nested type with the appropriate
-  // name. There are no conformance tables.
-  if (auto archetype = type->getAs<ArchetypeType>()) {
-    return archetype->getNestedType(name);
-  }
-
   // Find the named requirement.
   AssociatedTypeDecl *assocType = nullptr;
   auto members = conformance.getRequirement()->lookupDirect(name);
@@ -153,8 +147,15 @@ ProtocolConformanceRef::getTypeWitnessByName(Type type,
   if (!assocType)
     return nullptr;
 
-  if (conformance.isAbstract())
+  if (conformance.isAbstract()) {
+    // For an archetype, retrieve the nested type with the appropriate
+    // name. There are no conformance tables.
+    if (auto archetype = type->getAs<ArchetypeType>()) {
+      return archetype->getNestedType(name);
+    }
+
     return DependentMemberType::get(type, assocType);
+  }
 
   auto concrete = conformance.getConcrete();
   if (!concrete->hasTypeWitness(assocType, resolver)) {

test/Constraints/generic_super_constraint.swift

@@ -17,3 +17,11 @@ func bar<T, U>(_ x: U, y: T) -> (Derived, Int) where U: Base<T>, U: Derived {
   // expected-error@+1{{cannot convert return expression}}
   return (x, y)
 }
+
+// SR-7551 captures a crash on this code.
+class IntegerClass : ExpressibleByIntegerLiteral, Equatable {
+  required init(integerLiteral value: Int) { }
+  static func ==(lhs: IntegerClass, rhs: IntegerClass) -> Bool { return true }
+}
+
+func foo<T: IntegerClass>(_ num: T) { let x =  num != 0 }