[libcxx] [test] Avoid conflicting definitions of _CRT_SECURE_NO_WARNINGS

This is defined both by libcxx/utils/libcxx/test/config.py (for
any windows target) and msvc_stdlib_force_include.h (when testing
specifically the MSVC C++ library).

The command line define (-D_CRT_SECURE_NO_WARNINGS) defines it to the
value 1; change the header define to match that.

Keeping both instances, to keep the fix for cases when not building
in cases that don't use config.py.

Also remove a comment about whether this can be removed; it can't at
least be removed altogether - doing that breaks a number of tests that
otherwise succeed.

Differential Revision: https://reviews.llvm.org/D89588