Determine concurrency diagnostic behavior based on conformance context.

When determining whether to warn, error, or be silent about
concurrency-related issues detected between a protocol requirement and
its witness, decide based on the context of the conformance rather
than based on the context of the witness. Fixes rdar://88205585.

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -2908,7 +2908,7 @@ bool ConformanceChecker::checkActorIsolation(
   bool isCrossActor = false;
   bool witnessIsUnsafe = false;
   DiagnosticBehavior behavior = SendableCheckContext(
-      witness->getInnermostDeclContext()).defaultDiagnosticBehavior();
+      Conformance->getDeclContext()).defaultDiagnosticBehavior();
   Type witnessGlobalActor;
   switch (auto witnessRestriction =
               ActorIsolationRestriction::forDeclaration(

test/Concurrency/actor_isolation_unsafe.swift

@@ -29,10 +29,16 @@ struct S3_P1: P1 {
   nonisolated func onMainActor() { }
 }
 
+struct S4_P1_quitely: P1 {
+  @SomeGlobalActor func onMainActor() { }
+}
+
+@SomeGlobalActor
 struct S4_P1: P1 {
   @SomeGlobalActor func onMainActor() { } // expected-warning{{instance method 'onMainActor()' isolated to global actor 'SomeGlobalActor' can not satisfy corresponding requirement from protocol 'P1' isolated to global actor 'MainActor'}}
 }
 
+
 @MainActor(unsafe)
 protocol P2 {
   func f() // expected-note{{calls to instance method 'f()' from outside of its actor context are implicitly asynchronous}}

test/decl/class/actor/global_actor_conformance.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift  -disable-availability-checking
+// RUN: %target-typecheck-verify-swift  -disable-availability-checking -warn-concurrency
 // REQUIRES: concurrency
 
 actor SomeActor { }