[Diagnostics] Avoid adding comma when reordering with last argument

(cherry picked from commit e086896)

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -4825,9 +4825,14 @@ bool OutOfOrderArgumentFailure::diagnoseAsError() {
     SourceRange removalRange{Lexer::getLocForEndOfToken(SM, removalStartLoc),
                              firstRange.End};
 
+    // Move requires postfix comma only if argument is moved in-between
+    // other arguments.
+    bool requiresComma = !isExpr<BinaryExpr>(anchor) &&
+                         PrevArgIdx != tuple->getNumElements() - 1;
+
     diag.fixItRemove(removalRange);
     diag.fixItInsert(secondRange.Start,
-                     text.str() + (isExpr<BinaryExpr>(anchor) ? "" : ", "));
+                     text.str() + (requiresComma ? ", " : ""));
   };
 
   // There are 4 diagnostic messages variations depending on

test/Constraints/argument_matching.swift

@@ -1752,10 +1752,10 @@ func rdar70764991() {
     static var foo: S { get { S() } }
   }
 
-  func bar(_: Any, foo: String) {
+  func bar(_: Any, _: String) {
   }
 
   func test(_ str: String) {
-    bar(str, foo: S.foo) // expected-error {{unnamed argument #1 must precede argument 'foo'}}  {{9-12=}} {{14-14=str, }}
+    bar(str, S.foo) // expected-error {{unnamed argument #1 must precede unnamed argument #2}}  {{9-12=}} {{14-14=str}}
   }
 }