Allow `<coroutine>` to be included if `_DOWNLEVEL_COROUTINES_SUPPORTED`
is defined even if `__cpp_lib_coroutine` is not. Allows use of standard
coroutines in C++14 and C++17 modes with a compiler that permits it.
Comparison operators are explicitly defined instead of <=> in these
modes.

Corresponding compiler changes are expected to come in 16.10.

Add /permissive- to the /Zc:preprocessor config.

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

- Move detection of downlevel coroutines to yvals_core.h in
  definition of __cpp_lib_coroutine.
- Rework inclusion of <coroutine> in terms of __cpp_lib_coroutine,
guard <compare> inclusion and spaceship definition in _HAS_CXX20.
- C++14-ify the coroutine test.