Handle function name conflicts in _LIBCPP_MSVCRT mode

Visual Studio's SAL extension uses a macro named __deallocate. This macro is
used pervasively, and gets included through various different ways. This
conflicts with the similarly named interfaces in libc++. Introduce a undef
header similar to __undef_min_max to handle this. This fixes a number of errors
due to the macro replacing the function name.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@229162 91177308-0d34-0410-b5e6-96231b3b80d8

include/new

@@ -68,6 +68,8 @@ void  operator delete[](void* ptr, void*) noexcept;
 #include <exception>
 #include <cstddef>
 
+#include <__undef___deallocate>
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif