[libcxx] Portability fix: make std::io_base::xalloc() test independent on initial index value.

[amakc11]

Summary

The requirement to std::io_base::xalloc() is: "Returns: index++". The requirement to static field index is: "initialized to an unspecified value". This patch removes the dependency of the std::io_base::xalloc() test on specific initial value of this static field implemented in libc++ and makes the test pass on other conformant implementations.

test/std/input.output/iostreams.base/ios.base/ios.base.storage/xalloc.pass.cpp

 
 // static int xalloc();
 
 #include <ios>
 #include <cassert>
 
 int main(int, char**)
 {
-    assert(std::ios_base::xalloc() == 0);
-    assert(std::ios_base::xalloc() == 1);
-    assert(std::ios_base::xalloc() == 2);
-    assert(std::ios_base::xalloc() == 3);
-    assert(std::ios_base::xalloc() == 4);
+    int index = std::ios_base::xalloc();
+    for (int i = 0; i < 10000; ++i)
+        assert(std::ios_base::xalloc() == ++index);
 
   return 0;
 }