[Concurrency] Don't downgrade explicit isolation attribute clashes

The compiler shouldn't accept mismatch in explicit isolation attributes
because it could lead to incorrect isolation selection.

Resolves: rdar://155589753

(cherry picked from commit a9373c0)

Author: xedin

lib/Sema/TypeCheckAttr.cpp

@@ -4346,7 +4346,6 @@ static void checkGlobalActorAttr(
                    attributes[1])
         .highlight(attributes[0]->getRangeWithAt())
         .highlight(attributes[1]->getRangeWithAt())
-        .warnUntilSwiftVersion(6)
         .fixItRemove(attributes[1]->getRangeWithAt());
     return;
   }
@@ -4357,7 +4356,6 @@ static void checkGlobalActorAttr(
         .highlight(attributes[0]->getRangeWithAt())
         .highlight(attributes[1]->getRangeWithAt())
         .highlight(attributes[2]->getRangeWithAt())
-        .warnUntilSwiftVersion(6)
         .fixItRemove(attributes[1]->getRangeWithAt())
         .fixItRemove(attributes[2]->getRangeWithAt());
     return;
@@ -4370,7 +4368,6 @@ static void checkGlobalActorAttr(
       .highlight(attributes[1]->getRangeWithAt())
       .highlight(attributes[2]->getRangeWithAt())
       .highlight(attributes[3]->getRangeWithAt())
-      .warnUntilSwiftVersion(6)
       .fixItRemove(attributes[1]->getRangeWithAt())
       .fixItRemove(attributes[2]->getRangeWithAt())
       .fixItRemove(attributes[3]->getRangeWithAt());

test/Concurrency/isolated_parameters.swift

@@ -339,15 +339,15 @@ func isolatedClosures() {
   }
 }
 
-// expected-warning@+3 {{global function 'allOfEm' has multiple actor-isolation attributes (@MainActor and 'nonisolated')}}
-// expected-warning@+2 {{global function with 'isolated' parameter cannot be 'nonisolated'; this is an error in the Swift 6 language mode}}{{12-24=}}
-// expected-warning@+1 {{global function with 'isolated' parameter cannot have a global actor; this is an error in the Swift 6 language mode}}{{1-12=}}
+#if ALLOW_TYPECHECKER_ERRORS
+
+// expected-typechecker-error@+3 {{global function 'allOfEm' has multiple actor-isolation attributes (@MainActor and 'nonisolated')}}
+// expected-typechecker-warning@+2 {{global function with 'isolated' parameter cannot be 'nonisolated'; this is an error in the Swift 6 language mode}}{{12-24=}}
+// expected-typechecker-warning@+1 {{global function with 'isolated' parameter cannot have a global actor; this is an error in the Swift 6 language mode}}{{1-12=}}
 @MainActor nonisolated func allOfEm(_ a: isolated A) {
   a.f()
 }
 
-#if ALLOW_TYPECHECKER_ERRORS
-
 @MainActor class MAClass {
 
   // expected-typechecker-note@+2 {{previous 'isolated' parameter 'a'}}

test/attr/execution_behavior_attrs.swift

@@ -99,11 +99,11 @@ struct TestAttributeCollisions {
   }
 
   @MainActor @concurrent func testGlobalActor() async {}
-  // expected-warning @-1 {{instance method 'testGlobalActor()' has multiple actor-isolation attributes (@MainActor and @concurrent)}}
+  // expected-error@-1 {{instance method 'testGlobalActor()' has multiple actor-isolation attributes (@MainActor and @concurrent)}}
 
   nonisolated(nonsending) nonisolated func testNonIsolatedCaller() async {} // expected-error {{duplicate modifier}} expected-note {{modifier already specified here}}
   @MainActor nonisolated(nonsending) func testGlobalActorCaller() async {}
-  // expected-warning@-1 {{instance method 'testGlobalActorCaller()' has multiple actor-isolation attributes (@MainActor and 'nonisolated(nonsending)')}}
+  // expected-error@-1 {{instance method 'testGlobalActorCaller()' has multiple actor-isolation attributes (@MainActor and 'nonisolated(nonsending)')}}
   nonisolated(nonsending) func testCaller(arg: isolated MainActor) async {}
   // expected-error@-1 {{cannot use 'nonisolated(nonsending)' on instance method 'testCaller(arg:)' because it has an isolated parameter: 'arg'}}
 

test/attr/global_actor.swift

@@ -98,7 +98,7 @@ struct Container {
 // Redundant attributes
 // -----------------------------------------------------------------------
 extension SomeActor {
-  @GA1 nonisolated func conflict1() { } // expected-warning {{instance method 'conflict1()' has multiple actor-isolation attributes (@GA1 and 'nonisolated')}}
+  @GA1 nonisolated func conflict1() { } // expected-error {{instance method 'conflict1()' has multiple actor-isolation attributes (@GA1 and 'nonisolated')}}
 }