[libcxx] Fix rethrow_if_nested test on Windows.

[halyavin]

Summary

Stack overflow is caused by B copy constructor invoking default constructor of std::nested_exception which tries to capture current exception. Capturing current exception copies it and since our current exception is B, we call B's copy constructor and cause infinite recursion.

After fix, "throw b" no longer captures current exception (it copies it from B(5) object where it is empty) and so throw_if_nested terminates the test.

test/std/language.support/support.exception/except.nested/rethrow_if_nested.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: libcpp-no-exceptions
 
-// This test fails due to a stack overflow
-// XFAIL: LIBCXX-WINDOWS-FIXME
-
 // <exception>
 
 // class nested_exception;
 
 // template <class E> void rethrow_if_nested(const E& e);
 
 #include <exception>
 #include <cstdlib>

 };
 
 class B
     : public std::nested_exception,
       public A
 {
 public:
     explicit B(int data) : A(data) {}
-    B(const B& b) : A(b) {}
+    B(const B& b) : std::nested_exception(b), A(b) {}
 };
 
 class C
 {
 public:
     virtual ~C() {}
     C * operator&() const { assert(false); return nullptr; } // should not be called
 };
int main()
         try
         {
             throw B(5);
         }
         catch (const B& b)
         {
             try
             {
-                throw b;
+                throw B(4);
             }
             catch (const A& a)
             {
                 try
                 {
                     std::rethrow_if_nested(a);
                     assert(false);
                 }