Allow -preview=in only with extern(D|C++)

The intent of `-preview=in` is to make `in` the go-to storage class for input parameters in D.
However it is D centric, as it is an enhanced version of `scope const ref`.
As non-`extern(D)` functions usually are expected to match a specific ABI,
using `in` is hardly a good idea.

However, as C++, also have a "go to" storage class for input parameters (`const T&`),
`in` can also be applied on `extern(C++)` function in order to bind to `const T&` parameters.
This also allows to expose a closer API for a function than via `const ref`,
as `in` will allow to bind rvalues to `const T&`, as in C++.

test/runnable_cxx/extra-files/cppb.cpp

@@ -4,30 +4,8 @@
 #include <exception>
 #include <cstdarg>
 
-#if _WIN32 // otherwise defined in C header files!
-// https://issues.dlang.org/show_bug.cgi?id=18955
-namespace std
-{
-    template<typename Char>
-    struct char_traits
-    {
-    };
-    template<typename Char>
-    class allocator
-    {
-    };
-    template<typename Char, typename Traits, typename Alloc>
-    class basic_string
-    {
-    };
-    typedef basic_string<char, char_traits<char>, allocator<char> > string;
-}
-#else // if POSIX
-
 #include <string>
 
-#endif // _WIN32
-
 #include "cppb.h"
 
 /**************************************/
@@ -936,3 +914,11 @@ void test18955()
     callback18955(s);
 #endif
 }
+
+void previewInFunction(const int& a, const std::string& b, const std::string& c);
+
+void testPreviewIn()
+{
+    std::string s = "Hello World";
+    previewInFunction(42, s, s);
+}