[SwiftCompiler] Move common bridging facilities to 'Basic'

A preparation for AST/DiagnosticEngine bridging

include/swift/Basic/SwiftObjectHeader.h

@@ -0,0 +1,34 @@
+//===--- SwiftObjectHeader.h - Defines SwiftObjectHeader ------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+#ifndef SWIFT_BASIC_SWIFTOBJECTHEADER_H
+#define SWIFT_BASIC_SWIFTOBJECTHEADER_H
+
+#include "BridgedSwiftObject.h"
+
+/// The C++ version of SwiftObject.
+///
+/// It is used for bridging the SIL core classes (e.g. SILFunction, SILNode,
+/// etc.) with Swift.
+/// For details see SwiftCompilerSources/README.md.
+///
+/// In C++ code, never use BridgedSwiftObject directly. SwiftObjectHeader has
+/// the proper constructor, which avoids the header to be uninitialized.
+struct SwiftObjectHeader : BridgedSwiftObject {
+  SwiftObjectHeader(SwiftMetatype metatype) {
+    this->metatype = metatype;
+    this->refCounts = ~(uint64_t)0;
+  }
+
+  bool isBridged() const { return metatype != nullptr; }
+};
+
+#endif