AST: Print parameterized protocols in opaque return types

The primary change was the refactoring of ArchetypeType::getExistentialType();
this commit just fixes the type sugar.

Fixes rdar://problem/100911362.

Author: slavapestov

lib/AST/ASTPrinter.cpp

@@ -6466,6 +6466,13 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
       if (auto existential = constraint->getAs<ExistentialType>())
         constraint = existential->getConstraintType();
 
+      // Opaque archetype substitutions are always canonical, so re-sugar the
+      // constraint type using the owning declaration's generic parameter names.
+      auto genericSig = T->getDecl()->getNamingDecl()->getInnermostDeclContext()
+          ->getGenericSignatureOfContext();
+      if (genericSig)
+        constraint = genericSig->getSugaredType(constraint);
+
       visit(constraint);
       return;
     }

test/ModuleInterface/parameterized-protocol-opaque-result-types.swift

@@ -0,0 +1,19 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-emit-module-interface(%t/ParameterizedProtocols.swiftinterface) %s -module-name ParameterizedProtocols
+// RUN: %target-swift-typecheck-module-from-interface(%t/ParameterizedProtocols.swiftinterface) -module-name ParameterizedProtocols
+// RUN: %FileCheck %s < %t/ParameterizedProtocols.swiftinterface
+
+public protocol P<T> {
+  associatedtype T
+}
+
+struct S<T>: P {}
+
+// CHECK-LABEL: public func returnsP() -> some ParameterizedProtocols.P
+public func returnsP() -> some P { return S<Int>() }
+
+// CHECK-LABEL: public func returnsPInt() -> some ParameterizedProtocols.P<Swift.Int>
+public func returnsPInt() -> some P<Int> { return S<Int>() }
+
+// CHECK-LABEL: public func returnsPT<T>(_: T) -> some ParameterizedProtocols.P<T>
+public func returnsPT<T>(_: T) -> some P<T> { return S<T>() }