[Concurrency] Avoid Sendable checking for conversions that strip @Sendable from nonisolated(nonsending) function type

The isolation doesn't change in this cause and making function type
non-Sendable means that it woun't be able to leave the current concurrency
domain and the compiler won't generate a thunk.

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -2780,16 +2780,13 @@ namespace {
               diagnoseNonSendableParametersAndResult(toFnType);
               break;
 
-            case FunctionTypeIsolation::Kind::Parameter:
-              llvm_unreachable("invalid conversion");
-
+            // @Sendable nonisolated(nonsending) -> nonisolated(nonsending)
+            // doesn't require Sendable checking.
             case FunctionTypeIsolation::Kind::NonIsolatedNonsending:
-              // Non isolated caller is always async. This can only occur if we
-              // are converting from an `@Sendable` representation to something
-              // else. So we need to just check that we diagnose non sendable
-              // parameters and results.
-              diagnoseNonSendableParametersAndResult(toFnType);
               break;
+
+            case FunctionTypeIsolation::Kind::Parameter:
+              llvm_unreachable("invalid conversion");
             }
             break;
           }

test/Concurrency/attr_execution/conversions_silgen.swift

@@ -620,4 +620,12 @@ func testSendableClosureNonisolatedNonSendingInference() {
   // CHECK: hop_to_executor [[EXECUTOR]]
   // CHECK: // end sil function '$s21attr_execution_silgen49testSendableClosureNonisolatedNonSendingInferenceyyFySiYaYbYCcfU_'
   let _: nonisolated(nonsending) @Sendable (Int) async -> Void = { _ in }
+
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen49testSendableClosureNonisolatedNonSendingInferenceyyFyS2SYaKYCcYaYbYCcfU0_ : $@convention(thin) @Sendable @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>, @guaranteed @async @callee_guaranteed (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>, @guaranteed String) -> (@owned String, @error any Error)) -> @error any Error
+  // CHECK: bb0([[EXECUTOR:%.*]] : @guaranteed $Optional<any Actor>, %1 : @guaranteed $@async @callee_guaranteed (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>, @guaranteed String) -> (@owned String, @error any Error)):
+  // CHECK: hop_to_executor [[EXECUTOR]]
+  // CHECK: // end sil function '$s21attr_execution_silgen49testSendableClosureNonisolatedNonSendingInferenceyyFyS2SYaKYCcYaYbYCcfU0_'
+  let _: nonisolated(nonsending) @Sendable (
+    nonisolated(nonsending) @escaping (String) async throws -> String
+  ) async throws -> Void = { _ in }
 }