[libcxx] Fix deprecation warning by suppressing deprecated around

__test_has_construct.

In C++17 some tests started failing after a521532a. This fixes those errors by suppressing the deprecation warning when calling `construct` in `__test_has_construct`. This is the same solution as `__has_destroy_test` already uses.

Reviewers: ldionne, #libc!

Subscribers: dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D80481