[test-suite][prolangs] fix ambiguous reference to stack

Since llvm/llvm-project@7b46225
libc++ headers will now include std::stack in more places, meaning the local stack() functions
plus overbroad `using namespace std;` in this program cause ambiguous references.

Fix this by narrowing the using directive to only what is needed.

Author: daltenty

MultiSource/Benchmarks/Prolangs-C++/shapes/bjarne.cpp

@@ -2,7 +2,7 @@
    in Bjarne Stroustrup's book "The C++ Programming Language" 2nd Edition. */
 #include <iostream>
 #include "bjarne.h"
-using namespace std;
+using std::cout;
 
 enum color {black='*', white=' '};