[libc++] Tolerate presence of __deallocate macro

Summary:
On Windows the identifier `__deallocate` is defined as a macro by one of the Windows system headers. Previously libc++ worked around this by `#undef __deallocate` and generating a warning. However this causes the WIN32 version of `__threading_support` to always generate a warning on Windows. This is not OK.

This patch renames all usages of `__deallocate` internally as to not conflict with the macro.

Reviewers: mclow.lists, majnemer, rnk, rsmith, smeenai, compnerd

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28426

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291332 91177308-0d34-0410-b5e6-96231b3b80d8

src/experimental/memory_resource.cpp

@@ -34,7 +34,7 @@ protected:
         { return __allocate(__size); }
 
     virtual void do_deallocate(void * __p, size_t, size_t)
-        { __deallocate(__p); }
+        { _VSTD::__libcpp_deallocate(__p); }
 
     virtual bool do_is_equal(memory_resource const & __other) const _NOEXCEPT
         { return &__other == this; }