[SourceKit] Update demangle for new mangling

Use 'swift::Demangle::isSwiftSymbol()' instead of homemade function

rdar://problem/39286958

Author: rintaro

test/SourceKit/Demangle/demangle.swift

@@ -1,15 +1,17 @@
-// RUN: %sourcekitd-test -req=demangle unmangled _TtBf80_  _TtP3foo3bar_ | %FileCheck %s
+// RUN: %sourcekitd-test -req=demangle unmangled _TtBf80_  _TtP3foo3bar_ '$S3Foo11AppDelegateC29applicationDidFinishLaunchingyy10Foundation12NotificationVF' | %FileCheck %s
 // CHECK:      START DEMANGLE
 // CHECK-NEXT: <empty>
 // CHECK-NEXT: Builtin.Float80
 // CHECK-NEXT: foo.bar
+// CHECK-NEXT: Foo.AppDelegate.applicationDidFinishLaunching(Foundation.Notification) -> ()
 // CHECK-NEXT: END DEMANGLE
 
-// RUN: %sourcekitd-test -req=demangle unmangled _TtBf80_  _TtP3foo3bar_ -simplified-demangling | %FileCheck %s -check-prefix=SIMPLIFIED
+// RUN: %sourcekitd-test -req=demangle unmangled _TtBf80_  _TtP3foo3bar_ '$S3Foo11AppDelegateC29applicationDidFinishLaunchingyy10Foundation12NotificationVF' -simplified-demangling | %FileCheck %s -check-prefix=SIMPLIFIED
 // SIMPLIFIED:      START DEMANGLE
 // SIMPLIFIED-NEXT: <empty>
 // SIMPLIFIED-NEXT: Builtin.Float80
 // SIMPLIFIED-NEXT: bar
+// SIMPLIFIED-NEXT: AppDelegate.applicationDidFinishLaunching(_:)
 // SIMPLIFIED-NEXT: END DEMANGLE
 
 // RUN: %sourcekitd-test -req=mangle Foo.Baru Swift.Beer | %FileCheck %s -check-prefix=MANGLED

tools/SourceKit/tools/sourcekitd/lib/API/Requests.cpp

@@ -1230,13 +1230,6 @@ class SKDocConsumer : public DocInfoConsumer {
 };
 } // end anonymous namespace
 
-static bool isSwiftPrefixed(StringRef MangledName) {
-  if (MangledName.size() < 2)
-    return false;
-  return MangledName[0] == '_' &&
-         (MangledName[1] == 'T' || MangledName[1] == MANGLING_PREFIX_STR[1]);
-}
-
 static sourcekitd_response_t demangleNames(ArrayRef<const char *> MangledNames,
                                            bool Simplified) {
   swift::Demangle::DemangleOptions DemangleOptions;
@@ -1246,7 +1239,7 @@ static sourcekitd_response_t demangleNames(ArrayRef<const char *> MangledNames,
   }
 
   auto getDemangledName = [&](StringRef MangledName) -> std::string {
-    if (!isSwiftPrefixed(MangledName))
+    if (!swift::Demangle::isSwiftSymbol(MangledName))
       return std::string(); // Not a mangled name
 
     std::string Result = swift::Demangle::demangleSymbolAsString(
@@ -2753,4 +2746,4 @@ static void enableCompileNotifications(bool value) {
   } else {
     trace::unregisterConsumer(&compileConsumer);
   }
-}
+}