[NFC][interop] rename the PrintAsObjC library to PrintAsClang

We're starting to support emission of C++ header interfaces, so a language-agnostic name makes more sense

lib/PrintAsClang/ModuleContentsWriter.h

@@ -0,0 +1,39 @@
+//===--- ModuleContentsWriter.h - Walk a module to print ObjC ---*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 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_PRINTASCLANG_MODULECONTENTSWRITER_H
+#define SWIFT_PRINTASCLANG_MODULECONTENTSWRITER_H
+
+#include "swift/AST/AttrKind.h"
+#include "swift/Basic/LLVM.h"
+#include "llvm/ADT/PointerUnion.h"
+#include "llvm/ADT/SmallPtrSet.h"
+
+namespace clang {
+  class Module;
+}
+
+namespace swift {
+class ModuleDecl;
+
+using ImportModuleTy = PointerUnion<ModuleDecl*, const clang::Module*>;
+
+/// Prints the declarations of \p M to \p os and collecting imports in
+/// \p imports along the way.
+void printModuleContentsAsObjC(raw_ostream &os,
+                               llvm::SmallPtrSetImpl<ImportModuleTy> &imports,
+                               ModuleDecl &M);
+
+} // end namespace swift
+
+#endif
+