ASTPrinter: Remove printWithCompatibilityFeatureChecks() from printSynthesizedExtensionIfNeeded()

This path was not exercised in the test suite, and when the
PrimaryAssociatedTypes feature finally (incorrectly) triggered it,
it would print '#endifextension'.

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -650,16 +650,7 @@ public:
         printer << " {}";
       };
 
-      bool printedNewline = false;
-      if (printOptions.PrintCompatibilityFeatureChecks) {
-        printedNewline =
-          printWithCompatibilityFeatureChecks(printer, curPrintOptions,
-                                              proto, printBody);
-      } else {
       printBody();
-        printedNewline = false;
-      }
-      if (!printedNewline)
       printer << "\n";
     }
   }

test/ModuleInterface/parameterized-protocol-synthesized-extension.swift

@@ -0,0 +1,22 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -typecheck -module-name ParameterizedProtocols -emit-module-interface-path %t/ParameterizedProtocols.swiftinterface %s
+// RUN: %FileCheck %s < %t/ParameterizedProtocols.swiftinterface
+
+public struct S1 : P1 {
+  public typealias T = Int
+}
+
+public struct S2 : Q1 {}
+
+protocol P1 : P2 {}
+
+public protocol P2<T> {
+  associatedtype T
+}
+
+protocol Q1 : Q2 {}
+
+public protocol Q2 {}
+
+// CHECK: extension ParameterizedProtocols.S1 : ParameterizedProtocols.P2 {}
+// CHECK-NEXT: extension ParameterizedProtocols.S2 : ParameterizedProtocols.Q2 {}