WCDH: introduce BARE_FEATURES

This allows defining compat versions of some C/C++ features with the name of the
keyword itself, so all code can look as if it was written for the new language
standard.

Tests/Module/WriteCompilerDetectionHeader/main_bare.cpp

@@ -0,0 +1,23 @@
+#include "test_compiler_detection_bare_features.h"
+
+class base
+{
+public:
+  virtual ~base() {}
+  virtual void baz() = 0;
+};
+
+class foo final
+{
+public:
+  virtual ~foo() {}
+  char* bar;
+  void baz() noexcept override { bar = nullptr; }
+};
+
+int main()
+{
+  foo f;
+
+  return 0;
+}