[libcxx] Rejigger test for destroying delete feature-test macros

In r361572, we introduced library support for C++20 destroying delete
and decided to only define the library feature-test macro when the
compiler supports the underlying language feature. This patch reworks
the tests to mirror that.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@366263 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed