Don't apply "Convert to Trailing Closure" to call with multiple trailing closures

Previously we'd perform an invalid transform or hit
an assertion failure. For now, disallow the transform
for a call that already uses trailing closure syntax.

rdar://81106400

Author: hamishknight

lib/IDE/Refactoring.cpp

@@ -3709,7 +3709,7 @@ static CallExpr *findTrailingClosureTarget(
     return nullptr;
   CallExpr *CE = cast<CallExpr>(contexts.back().get<Expr*>());
 
-  if (CE->hasTrailingClosure())
+  if (CE->getUnlabeledTrailingClosureIndex().hasValue())
     // Call expression already has a trailing closure.
     return nullptr;
 

test/refactoring/TrailingClosure/rdar81106400.swift

@@ -0,0 +1,9 @@
+func lottaClosures(x: () -> Void, y: () -> Void, z: () -> Void) {
+  lottaClosures(x: {}) {} z: {}
+}
+// RUN: not %refactor -trailingclosure -source-filename %s -pos=2:3
+
+func singleClosure(x: () -> Void) {
+  singleClosure {}
+}
+// RUN: not %refactor -trailingclosure -source-filename %s -pos=7:3