[libcxx] [test] Improve portability of vector tests.

[STL_MSFT]

Summary

Improve portability of vector tests.

This test was assuming libcxx's 2x growth factor, which isn't guaranteed by the Standard and isn't provided by MSVC (we use 1.5x). As a result the exact capacity check fails. Marking the assertion as libcxx-specific is all we need, and allows the rest of the test to succeed.

[EricWF]

r272745.

test/std/containers/sequences/vector/vector.capacity/shrink_to_fit.pass.cpp

int main()
         assert(is_contiguous_container_asan_correct(v));
     }
 #ifndef _LIBCPP_NO_EXCEPTIONS
     {
         std::vector<int, stack_allocator<int, 400> > v(100);
         v.push_back(1);
         assert(is_contiguous_container_asan_correct(v));
         v.shrink_to_fit();
-        assert(v.capacity() == 200);
+        LIBCPP_ASSERT(v.capacity() == 200); // assumes libc++'s 2x growth factor
         assert(v.size() == 101);
         assert(is_contiguous_container_asan_correct(v));
     }
 #endif
 #if TEST_STD_VER >= 11
     {
         std::vector<int, min_allocator<int>> v(100);
         v.push_back(1);
         assert(is_contiguous_container_asan_correct(v));
         v.shrink_to_fit();
         assert(v.capacity() == 101);
         assert(v.size() == 101);
         assert(is_contiguous_container_asan_correct(v));
     }
 #endif
 }