Merge pull request #65589 from hyp/eng/bridging-header-fixups

[interop] update <swift/bridging> to have language comment and __has_attribute guard

Author: hyp

lib/ClangImporter/bridging

@@ -1,3 +1,4 @@
+// -*- C++ -*-
 //===------------------ bridging - C++ and Swift Interop --------*- C++ -*-===//
 //
 // This source file is part of the Swift.org open source project
@@ -17,6 +18,14 @@
 #ifndef SWIFT_CLANGIMPORTER_SWIFT_INTEROP_SUPPORT_H
 #define SWIFT_CLANGIMPORTER_SWIFT_INTEROP_SUPPORT_H
 
+#ifdef __has_attribute
+#define _CXX_INTEROP_HAS_ATTRIBUTE(x) __has_attribute(x)
+#else
+#define _CXX_INTEROP_HAS_ATTRIBUTE(x) 0
+#endif
+
+#if _CXX_INTEROP_HAS_ATTRIBUTE(swift_attr)
+
 /// Specifies that a C++ `class` or `struct` owns and controls the lifetime of all
 /// of the objects it references. Such type should not reference any objects whose
 /// lifetime is controlled externally. This annotation allows Swift to import methods
@@ -124,4 +133,20 @@
 #define SWIFT_COMPUTED_PROPERTY \
   __attribute__((swift_attr("import_computed_property")))
 
+#else  // #if _CXX_INTEROP_HAS_ATTRIBUTE(swift_attr)
+
+// Empty defines for compilers that don't support `attribute(swift_attr)`.
+#define SWIFT_SELF_CONTAINED
+#define SWIFT_RETURNS_INDEPENDENT_VALUE
+#define SWIFT_SHARED_REFERENCE(_retain, _release)
+#define SWIFT_IMMORTAL_REFERENCE
+#define SWIFT_UNSAFE_REFERENCE
+#define SWIFT_NAME(_name)
+#define SWIFT_CONFORMS_TO_PROTOCOL(_moduleName_protocolName)
+#define SWIFT_COMPUTED_PROPERTY
+
+#endif // #if _CXX_INTEROP_HAS_ATTRIBUTE(swift_attr)
+
+#undef _CXX_INTEROP_HAS_ATTRIBUTE
+
 #endif // SWIFT_CLANGIMPORTER_SWIFT_INTEROP_SUPPORT_H