Mangling: fix mangling of functions with a single empty tuple argument

For example:
	func testit(_ x: ()) -> Int

Author: eeckstein

lib/AST/ASTMangler.cpp

@@ -1351,11 +1351,13 @@ void ASTMangler::appendParams(Type ParamsTy, bool forceSingleParam) {
     if (Tuple->getNumElements() == 0) {
       if (forceSingleParam) {
         // A tuple containing a single empty tuple.
+        appendOperator("y");
         appendOperator("t");
         appendListSeparator();
         appendOperator("t");
+      } else {
+        appendOperator("y");
       }
-      appendOperator("y");
       return;
     }
     if (forceSingleParam && Tuple->getNumElements() > 1) {

test/Demangle/Inputs/manglings.txt

@@ -251,3 +251,5 @@ _T08mangling14varargsVsArrayySaySiG3arrd_tF ---> mangling.varargsVsArray(arr: Sw
 _T0s13_UnicodeViewsVss22RandomAccessCollectionRzs0A8EncodingR_11SubSequence_5IndexQZAFRtzsAcERpzAE_AEQZAIRSs15UnsignedInteger8Iterator_7ElementRPzAE_AlMQZANRS13EncodedScalar_AlMQY_AORSr0_lE13CharacterViewVyxq__G ---> (extension in Swift):Swift._UnicodeViews<A, B><A, B where A: Swift.RandomAccessCollection, B: Swift.UnicodeEncoding, A.Index == A.SubSequence.Index, A.SubSequence: Swift.RandomAccessCollection, A.SubSequence == A.SubSequence.SubSequence, A.Iterator.Element: Swift.UnsignedInteger, A.Iterator.Element == A.SubSequence.Iterator.Element, A.SubSequence.Iterator.Element == B.EncodedScalar.Iterator.Element>.CharacterView
 _T010Foundation11MeasurementV12SimulatorKitSo9UnitAngleCRszlE11OrientationO2eeoiSbAcDEAGOyAF_G_AKtFZ ---> static (extension in SimulatorKit):Foundation.Measurement<A where A == __C.UnitAngle>.Orientation.== infix((extension in SimulatorKit):Foundation.Measurement<__C.UnitAngle>.Orientation, (extension in SimulatorKit):Foundation.Measurement<__C.UnitAngle>.Orientation) -> Swift.Bool
 _T04main1_yyF ---> main._() -> ()
+_T04test6testitSiyt_tF ---> test.testit(()) -> Swift.Int
+

test/SILGen/arguments_as_tuple_overloads.swift

@@ -28,7 +28,7 @@ public func test(_ a: Int, _ b: Int) {
 public func test(_ t: (Int, Int)) {
 }
 
-// CHECK: sil @_T04test3fooyt_tyF :
+// CHECK: sil @_T04test3fooyyt_tF :
 public func foo(_: ()) {
 }