WCDH: Test that no C compiler features are defined for CXX compiler.

This amends commit 54156d72 (WCDH: Generate per-language files
in multi-file mode., 2015-01-17).

Tests/Module/WriteCompilerDetectionHeader/main.cpp

@@ -4,6 +4,10 @@
 #define PREFIX TEST
 #include "compile_tests.h"
 
+#ifdef TEST_COMPILER_C_STATIC_ASSERT
+#error Expect no C features defined
+#endif
+
 int main()
 {
   return 0;