[libcxx] [test] Fix mismatches between _aligned_malloc and free() on Windows

[mstorsjo]

Summary

This allows getting rid of one case of LIBCXX-WINDOWS-FIXME. The fixme
comment was inaccurate; these functions are provided these days, but the
test kept failing as it was using mismatched allocation and free
functions.

A similar issue was fixed earlier, in
6596778b46ba69517191e7397289228168064ff4. That test was fixed by
overriding the aligned operator new too, and returning a dummy fixed
allocation instead. As this test is libcxx specific, it can use the
internal std::__libcpp_aligned_free() instead, to match libcxx's
internal aligned operator new.

libcxx/test/libcxx/language.support/support.dynamic/libcpp_deallocate.sh.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // test libc++'s implementation of align_val_t, and the relevant new/delete
 // overloads in all dialects when -faligned-allocation is present.
 
-// Libc++ defers to the underlying MSVC library to provide the new/delete
-// definitions, which does not yet provide aligned allocation
-// XFAIL: LIBCXX-WINDOWS-FIXME
-
 // XFAIL: LIBCXX-AIX-FIXME
 
 // The dylibs shipped before macosx10.13 do not contain the aligned allocation
 // functions, so trying to force using those with -faligned-allocation results
 // in a link error.
 // XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
 
 // Libcxx when built for z/OS doesn't contain the aligned allocation functions,
void operator delete(void* p, size_t n)TEST_NOEXCEPT {
   stats.sized_called++;
   stats.last_size = n;
   stats.last_align = -1;
 }
 #endif
 
 #ifndef NO_ALIGN
 void operator delete(void* p, std::align_val_t a)TEST_NOEXCEPT {
-  ::free(p);
+  std::__libcpp_aligned_free(p);
   stats.aligned_called++;
   stats.last_align = static_cast<int>(a);
   stats.last_size = -1;
 }
 
 void operator delete(void* p, size_t n, std::align_val_t a)TEST_NOEXCEPT {
-  ::free(p);
+  std::__libcpp_aligned_free(p);
   stats.aligned_sized_called++;
   stats.last_align = static_cast<int>(a);
   stats.last_size = n;
 }
 #endif
 
 void test_libcpp_dealloc() {
   void* p = nullptr;