[Concurrency] Forego Sendable checking if conversion doesn't change the global actor

`FunctionConversionExpr` is allowed to modify different attributes
of a type, sometimes it could strip `@Sendable` but keep the same
global actor attribute in place, that needs to be handled explicitly
before performing Sendable checking because in this case there is
going to be no isolation context change for arguments or results.

Resolves: rdar://153646123

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -2849,6 +2849,14 @@ namespace {
               break;
 
             case FunctionTypeIsolation::Kind::GlobalActor:
+              // If the isolation is the same it means that conversion
+              // covers loss of `@Sendable` or some other attribute and
+              // we don't need Sendable checking because there is no
+              // boundary crossing here.
+              if (fromIsolation.getGlobalActorType()->isEqual(
+                      toIsolation.getGlobalActorType()))
+                break;
+
               diagnoseNonSendableParametersAndResult(
                   toFnType, version::Version::getFutureMajorLanguageVersion());
               break;

test/Concurrency/dynamic_checks_for_func_refs_in_preconcurrency_apis.swift

@@ -42,6 +42,10 @@ public struct S<T> {
   }
 }
 
+public class NS {}
+
+public func testNoIsolation(_: @escaping (NS) -> Void) {}
+
 //--- Client.swift
 import API
 
@@ -164,3 +168,14 @@ func testMembers<X>(v: X, s: S<X>, fn: @escaping @MainActor (X) -> Int) {
 // CHECK-NEXT: [[EXEC:%.*]] = extract_executor [[MAIN_ACTOR_ACCESS]] : $MainActor
 // CHECK: [[CHECK_EXEC_REF:%.*]] = function_ref @$ss22_checkExpectedExecutor14_filenameStart01_D6Length01_D7IsASCII5_line9_executoryBp_BwBi1_BwBetF
 // CHECK-NEXT: {{.*}} = apply [[CHECK_EXEC_REF]]({{.*}}, [[EXEC]])
+
+@MainActor
+class Test {
+  @Sendable func compute(_: NS) {}
+  // expected-warning@-1 {{main actor-isolated synchronous instance method 'compute' cannot be marked as '@Sendable'; this is an error in the Swift 6 language mode}}
+
+  func test() {
+    // Triggers loss of @Sendable and actor isolation erasure at the same time
+    testNoIsolation(compute)
+  }
+}