Allow to generate associated constants in the body of C++ structs.

Opt-in since it uses a C++17 feature, but this allow exactly the same usage in
C++ and Rust, which I think is nice.

This also fixes constants of transparent structs in general, since the initial
version of this patch broke a test (associated constants in enums), so I added a
test for that too.

tests/expectations/const_transparent.cpp

@@ -0,0 +1,7 @@
+#include <cstdarg>
+#include <cstdint>
+#include <cstdlib>
+
+using Transparent = uint8_t;
+
+static const Transparent FOO = 0;