[AST/Sema] Allow Sendable suppression on Objective-C class declarations

Expressed as `__swift_attr__("~Sendable")` this acts like `: ~Sendable`
on Swift type declarations and supersedes `@_nonSendable(_assumed)`.

Resolves: rdar://140928937

test/Concurrency/tilde_sendable_objc.swift

@@ -0,0 +1,47 @@
+// RUN: %empty-directory(%t/src)
+// RUN: %empty-directory(%t/sdk)
+// RUN: split-file %s %t/src
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck %t/src/main.swift \
+// RUN:   -import-objc-header %t/src/Test.h \
+// RUN:   -swift-version 6 \
+// RUN:   -enable-experimental-feature TildeSendable \
+// RUN:   -module-name main -I %t -verify -verify-ignore-unrelated
+
+// REQUIRES: objc_interop
+// REQUIRES: swift_feature_TildeSendable
+
+//--- Test.h
+#define SWIFT_SENDABLE __attribute__((__swift_attr__("@Sendable")))
+#define SWIFT_NONSENDABLE_ASSUMED __attribute__((__swift_attr__("@_nonSendable(_assumed)")))
+#define SWIFT_SUPPRESS_SENDABLE __attribute__((__swift_attr__("~Sendable")))
+
+@import Foundation;
+
+// Test that `~Sendable` superseeds `@_nonSendable(_assumed)` on classes.
+
+SWIFT_NONSENDABLE_ASSUMED
+SWIFT_SUPPRESS_SENDABLE
+@interface Parent : NSObject
+@end
+
+// Test that `Sendable` superseeds `@_nonSendable(_assumed)` and `~Sendable` from the parent.
+
+SWIFT_NONSENDABLE_ASSUMED
+SWIFT_SENDABLE
+@interface SendableValue : Parent
+@end
+
+SWIFT_NONSENDABLE_ASSUMED
+@interface NonSendableValue : Parent
+@end
+
+//--- main.swift
+func testSendable<T: Sendable>(_: T) {}
+
+public func test(p: Parent, v: SendableValue, ns: NonSendableValue) {
+  testSendable(p) // expected-error {{type 'Parent' does not conform to the 'Sendable' protocol}}
+  testSendable(v) // Ok (no diagnostics unable unavailable conformance associated with `Parent`).
+  testSendable(ns) // expected-error {{conformance of 'NonSendableValue' to 'Sendable' is unavailable}}
+}
+