replace `std::variant` with `__variant` in `as_awaitable` #2852

Job	Run time
CPU (Windows) (msvc, Release)	14m 42s
CPU (Windows) (msvc, Debug)	12m 42s
macos-CPU (MacOS) (clang, Release)	3m 32s
macos-CPU (MacOS) (clang, Debug)	4m 8s
CPU (gcc 11, Debug)	4m 15s
CPU (clang 16, Release, ASAN)	4m 57s
CPU (clang 16, Debug)	4m 36s
CPU (gcc 13, Debug)	4m 20s
CPU (clang 16, Release)	4m 53s
CPU (gcc 11, Release)	3m 59s
CPU (gcc 14, Release, c++23)	4m 39s
CPU (gcc 14, Release, LEAK)	4m 35s
CPU (gcc 12, Release, TSAN)	12m 57s
CPU (gcc 11, Release, ASAN)	4m 36s
CPU (clang 16, Debug, c++23)	12m 52s
CI (CPU) (MacOS)	0s
CI (CPU)	-1s
CI (CPU) (Windows)	0s
	1h 41m 42s

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