[TypeChecker] Downgrade noasync availability to warnings on @preconcurrency declarations

This change helps to stage in new `async` overloads in Swift 6 language mode
if `@preconcurrency` declaration is not available from async contexts.

Author: xedin

lib/Sema/TypeCheckAvailability.cpp

@@ -4147,6 +4147,8 @@ diagnoseDeclAsyncAvailability(const ValueDecl *D, SourceRange R,
     auto diag = ctx.Diags.diagnose(diagLoc, diag::async_unavailable_decl, D,
                                    attr->getMessage());
     diag.warnUntilSwiftVersion(6);
+    diag.limitBehaviorWithPreconcurrency(DiagnosticBehavior::Warning,
+                                         D->preconcurrency());
 
     if (!attr->getRename().empty()) {
       fixItAvailableAttrRename(diag, R, D, attr->getRename(), call);

test/Concurrency/unavailable_from_async_swift6.swift

@@ -0,0 +1,20 @@
+// RUN: %target-typecheck-verify-swift -target %target-swift-5.1-abi-triple -swift-version 6
+
+// REQUIRES: concurrency
+// REQUIRES: asserts
+
+struct API {
+  @available(*, noasync, message: "use complete() instead")
+  func wait() {}
+
+  @preconcurrency
+  @available(*, noasync, message: "use complete() instead")
+  func waitUntilComplete() {}
+
+  func complete() async {}
+}
+
+func test(v: API) async {
+  v.wait() // expected-error {{instance method 'wait' is unavailable from asynchronous contexts; use complete() instead}}
+  v.waitUntilComplete() // expected-warning {{instance method 'waitUntilComplete' is unavailable from asynchronous contexts; use complete() instead}}
+}