AST: Introduce a SemanticAvailableAttr::isNoAsync() convenience.

Author: tshortli

include/swift/AST/Attr.h

@@ -3197,6 +3197,9 @@ class SemanticAvailableAttr final {
   /// `PlatformKind::none` if the attribute is not platform specific.
   PlatformKind getPlatformKind() const { return getDomain().getPlatformKind(); }
 
+  /// Whether this is a `noasync` attribute.
+  bool isNoAsync() const { return attr->isNoAsync(); }
+
   /// Whether this attribute has an introduced, deprecated, or obsoleted
   /// version.
   bool isVersionSpecific() const {

lib/AST/Availability.cpp

@@ -588,8 +588,7 @@ const AvailableAttr *Decl::getNoAsyncAttr() const {
        getSemanticAvailableAttrs(/*includingInactive=*/false)) {
     auto attr = semanticAttr.getParsedAttr();
 
-    if (attr->getPlatformAgnosticAvailability() !=
-        PlatformAgnosticAvailabilityKind::NoAsync)
+    if (!semanticAttr.isNoAsync())
       continue;
 
     if (!bestAttr) {

lib/AST/Decl.cpp

@@ -9278,16 +9278,14 @@ AbstractFunctionDecl *AbstractFunctionDecl::getAsyncAlternative() const {
   // `getAttrs` is in reverse source order, so the last attribute is the
   // first in source.
   AbstractFunctionDecl *alternative = nullptr;
-  for (auto semanticAttr : getSemanticAvailableAttrs()) {
-    auto attr = semanticAttr.getParsedAttr();
-
-    if (attr->isNoAsync())
+  for (auto attr : getSemanticAvailableAttrs()) {
+    if (attr.isNoAsync())
       continue;
 
-    if (attr->getPlatform() != PlatformKind::none && alternative != nullptr)
+    if (attr.isPlatformSpecific() && alternative != nullptr)
       continue;
 
-    if (auto *renamedDecl = getRenamedDecl(attr)) {
+    if (auto *renamedDecl = getRenamedDecl(attr.getParsedAttr())) {
       if (auto *afd = dyn_cast<AbstractFunctionDecl>(renamedDecl)) {
         if (afd->hasAsync())
           alternative = afd;