Update __cpp_lib_coroutine

[StephanTLavavej]

Works towards #40.

Now that Victor Tong has implemented symmetric coroutine transfer for VS 2019 16.8 Preview 3 (Microsoft-internal MSVC-PR-267685), we can activate the library's feature-test macro.

Because Clang and EDG don't define __cpp_impl_coroutine yet, I'm structuring this in a cautious way. If we're in C++20 mode, and the compiler defines __cpp_impl_coroutine (true for MSVC right now), we'll inspect the value - if it's the Standard value (or newer, just in case), we'll define the library macro to the Standard value (which happens to be the same). If it's a pre-Standard value (true for MSVC as of VS 2019 16.8 Preview 1 right now, will probably be true until VS 2019 16.8 Preview 4), we'll define the library macro to the pre-Standard value of Nulluary 1970.

I'm updating the feature-test macro test accordingly. This adaptive behavior should allow the tests to continue passing as the compilers light up their impl macros. Adaptive behavior has been problematic in the past when we use a feature-test macro internally to activate code, but here it is precisely targeted at the <coroutine> header which is currently a pure leaf.

I'm also removing the "partially implemented" comment, since we've merged noop coroutines. However, #40 will remain open until we add end-to-end test coverage for MSVC. (At that time we'll probably spin off separate issues for the remaining checkboxes.)

[CaseyCarter]

We've been co_awaiting completion of the <coroutine> support for quite a while now, it's nice to see this work co_yield such great results. 😎