Replace existing access control in objcImpl fixit

The fix-it suggesting that an unmatched `@objc`-able member can be turned `private` always suggested adding a new modifier, even if one already existed. Make it suggest replacing the existing one instead.

Author: beccadax

lib/Sema/TypeCheckDeclObjC.cpp

@@ -3542,10 +3542,16 @@ class ObjCImplementationChecker {
       diagnose(cand, diag::member_of_objc_implementation_not_objc_or_final,
                cand, cand->getDeclContext()->getSelfClassDecl());
 
-      if (canBeRepresentedInObjC(cand))
-        diagnose(cand, diag::fixit_add_private_for_objc_implementation,
-                 cand->getDescriptiveKind())
-            .fixItInsert(cand->getAttributeInsertionLoc(true), "private ");
+      if (canBeRepresentedInObjC(cand)) {
+        auto diagnostic =
+            diagnose(cand, diag::fixit_add_private_for_objc_implementation,
+                     cand->getDescriptiveKind());
+        if (auto modifier = cand->getAttrs().getAttribute<AccessControlAttr>())
+          diagnostic.fixItReplace(modifier->getRange(), "private");
+        else
+          diagnostic.fixItInsert(cand->getAttributeInsertionLoc(true),
+                                 "private ");
+      }
 
       diagnose(cand, diag::fixit_add_final_for_objc_implementation,
                cand->getDescriptiveKind())

test/decl/ext/objc_implementation.swift

@@ -114,7 +114,7 @@ protocol EmptySwiftProto {}
 
   internal var propertyNotFromHeader1: CInt
   // expected-warning@-1 {{property 'propertyNotFromHeader1' does not match any property declared in the headers for 'ObjCClass'; did you use the property's Swift name?; this will become an error before '@_objcImplementation' is stabilized}}
-  // expected-note@-2 {{add 'private' or 'fileprivate' to define an Objective-C-compatible property not declared in the header}} {{3-3=private }}
+  // expected-note@-2 {{add 'private' or 'fileprivate' to define an Objective-C-compatible property not declared in the header}} {{3-11=private}}
   // expected-note@-3 {{add 'final' to define a Swift property that cannot be overridden}} {{3-3=final }}
 
   @objc private var propertyNotFromHeader2: CInt