[Runtime] Use the old remangler to compute the @objc protocol name.

The “old” mangling that is used for runtime names of @objc protocols
uses a simpler substitution scheme, so you can’t simply take a mangled
name from the new mangling and fix up the ends.

Fixes rdar://problem/45685649.

stdlib/public/runtime/MetadataLookup.cpp

@@ -946,8 +946,7 @@ public:
 #if SWIFT_OBJC_INTEROP
     // Look for a Swift-defined @objc protocol with the Swift 3 mangling that
     // is used for Objective-C entities.
-    std::string objcMangledName =
-      "_TtP" + mangledName.substr(0, mangledName.size()-1) + "_";
+    std::string objcMangledName = "_Tt" + mangleNodeOld(node) + "_";
     if (auto protocol = objc_getProtocol(objcMangledName.c_str()))
       return ProtocolDescriptorRef::forObjC(protocol);
 #endif

test/Runtime/demangleToMetadataObjC.swift

@@ -15,6 +15,7 @@ let DemangleToMetadataTests = TestSuite("DemangleToMetadataObjC")
 @objc protocol P1 { }
 protocol P2 { }
 @objc protocol P3: P1 { }
+@objc protocol mainP4 { }
 
 DemangleToMetadataTests.test("@objc classes") {
   expectEqual(type(of: C()), _typeByMangledName("4main1CC")!)
@@ -25,10 +26,13 @@ DemangleToMetadataTests.test("@objc enums") {
 }
 
 func f1_composition_objc_protocol(_: P1) { }
+func f1_composition_objc_protocol_P4(_: mainP4) { }
 
 DemangleToMetadataTests.test("@objc protocols") {
   expectEqual(type(of: f1_composition_objc_protocol),
               _typeByMangledName("yy4main2P1_pc")!)
+  expectEqual(type(of: f1_composition_objc_protocol_P4),
+              _typeByMangledName("yy4main0A2P4_pc")!)
 }
 
 DemangleToMetadataTests.test("Objective-C classes") {