Define CPPCORO_COMPILER_SUPPORTS_SYMMETRIC_TRANSFER for gcc and modern MSVC

[RedBeard0531]

MSVC now supports symmetric transfer, and I think gcc has since it introduced coroutines support. However, cppcoro currently only uses symmetric transfer for clang >= 7.

cppcoro/include/cppcoro/config.hpp

Lines 33 to 44 in 3912152

	/// \def CPPCORO_COMPILER_SUPPORTS_SYMMETRIC_TRANSFER
	/// Defined to 1 if the compiler supports returning a coroutine_handle from
	/// the await_suspend() method as a way of transferring execution
	/// to another coroutine with a guaranteed tail-call.
	#if CPPCORO_COMPILER_CLANG
	# if __clang_major__ >= 7
	# define CPPCORO_COMPILER_SUPPORTS_SYMMETRIC_TRANSFER 1
	# endif
	#endif
	#ifndef CPPCORO_COMPILER_SUPPORTS_SYMMETRIC_TRANSFER
	# define CPPCORO_COMPILER_SUPPORTS_SYMMETRIC_TRANSFER 0
	#endif

Rather than hard-coding a bunch more compiler versions, it may make more sense to check for __cpp_impl_coroutine >= 201902 to automatically use symmetric transfer for any compiler that claims compliant support of the final coroutine spec (probably with || __clang_minor__ >= 7 to keep working with old clang).

[RedBeard0531]

FYI, I ran into this while comparing codegen between gcc and clang on godbolt, where cppcoro is an easy way to quickly play with coroutines. When doing this I couldn't make sense of the gcc codegen until I realized that it was actually being fed different code than clang. When I added -DCPPCORO_COMPILER_SUPPORTS_SYMMETRIC_TRANSFER to the command line, everything made sense again. Hopefully nobody else has to go through this fun discovery process.