[cxx-interop] NFC: use a CxxShim name constant

Author: egorzhdan

lib/ClangImporter/ClangImporter.cpp

@@ -4157,7 +4157,7 @@ void ClangModuleUnit::getImportedModulesForLookup(
     // definitions of Swift protocols that C++ types might conform to, such as
     // CxxSequence.
     if (owner.SwiftContext.LangOpts.EnableCXXInterop &&
-        requiresCPlusPlus(clangModule) && clangModule->Name != "CxxShim") {
+        requiresCPlusPlus(clangModule) && clangModule->Name != CXX_SHIM_NAME) {
       auto *cxxModule =
           owner.SwiftContext.getModuleByIdentifier(owner.SwiftContext.Id_Cxx);
       if (cxxModule)
@@ -4733,7 +4733,7 @@ DeclRefExpr *getInteropStaticCastDeclRefExpr(ASTContext &ctx,
   }
 
   // Lookup our static cast helper function in the C++ shim module.
-  auto wrapperModule = ctx.getLoadedModule(ctx.getIdentifier("CxxShim"));
+  auto wrapperModule = ctx.getLoadedModule(ctx.getIdentifier(CXX_SHIM_NAME));
   assert(wrapperModule &&
          "CxxShim module is required when using members of a base class. "
          "Make sure you `import CxxShim`.");