Sema: Abstracted service for @cdecl style attributes

include/swift/AST/Decl.h

@@ -8172,6 +8172,11 @@ public:
   /// instance method.
   bool isObjCInstanceMethod() const;
 
+  /// Get the foreign language targeted by a @cdecl-style attribute, if any.
+  /// Used to abstract away the change in meaning of @cdecl vs @_cdecl while
+  /// formalizing the attribute.
+  std::optional<ForeignLanguage> getCDeclKind() const;
+
   /// Determine whether the name of an argument is an API name by default
   /// depending on the function context.
   bool argumentNameIsAPIByDefault() const;

lib/AST/Decl.cpp

@@ -10615,6 +10615,15 @@ bool AbstractFunctionDecl::isObjCInstanceMethod() const {
   return isInstanceMember() || isa<ConstructorDecl>(this);
 }
 
+std::optional<ForeignLanguage> AbstractFunctionDecl::getCDeclKind() const {
+  auto attr = getAttrs().getAttribute<CDeclAttr>();
+  if (!attr)
+    return std::nullopt;
+
+  return attr->Underscored ? ForeignLanguage::ObjectiveC
+                           : ForeignLanguage::C;
+}
+
 bool AbstractFunctionDecl::needsNewVTableEntry() const {
   auto &ctx = getASTContext();
   return evaluateOrDefault(