[concurrency] Change execution to override inferred global actor isolation to match nonisolated.

Specifically, we were attempting to diagnose cases like the following:

```swift
@mainactor protocol P {
  func foo() async
}

struct S : P {
  @execution(concurrent) func foo() async {}
}
```

This was just an attempt to be more conservative. After some conversations, it
came up that nonisolated does not work that way... that is the compiler will
accept the following and just hop in the protocol witness thunk:

```swift
@mainactor protocol P {
  func foo() async
}

struct S : P {
  nonisolated func foo() async {}
}
```

Author: gottesmm

lib/Sema/TypeCheckAttr.cpp

@@ -306,18 +306,6 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
         }
       }
 
-      // We need isolation check here because global actor isolation
-      // could be inferred.
-
-      auto isolation = getActorIsolation(F);
-      if (isolation.isGlobalActor()) {
-        diagnoseAndRemoveAttr(
-            attr,
-            diag::attr_execution_concurrent_incompatible_with_global_actor, F,
-            isolation.getGlobalActor());
-        return;
-      }
-
       break;
     }
 

test/attr/attr_execution.swift

@@ -51,7 +51,6 @@ struct TestAttributeCollisions {
   // expected-error@-1 {{cannot use '@execution(concurrent)' on instance method 'testIsolationAny(arg:)' because it has a dynamically isolated parameter: 'arg'}}
 
   @MainActor @execution(concurrent) func testGlobalActor() async {}
-  // expected-error@-1 {{cannot use '@execution(concurrent)' on instance method 'testGlobalActor()' isolated to global actor 'MainActor'}}
 
   @execution(concurrent) @Sendable func test(_: @Sendable () -> Void, _: sending Int) async {} // Ok
 }
@@ -63,11 +62,9 @@ protocol P {
 
 struct InfersMainActor : P {
   @execution(concurrent) func test() async {}
-  // expected-error@-1 {{cannot use '@execution(concurrent)' on instance method 'test()' isolated to global actor 'MainActor'}}
 }
 
 @MainActor
 struct IsolatedType {
   @execution(concurrent) func test() async {}
-  // expected-error@-1 {{cannot use '@execution(concurrent)' on instance method 'test()' isolated to global actor 'MainActor'}}
 }