[libcxx] [test] Avoid unary_function.

Replace unary_function inheritance (which was never required,
even in C++98) with argument_type and result_type typedefs.
This increases portability, as unary_function was removed in C++17
and MSVC has implemented that removal.

Fixes D45596.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@329974 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed