Make `opt::flatten` return reference options sometimes to make it mor… #975

Job	Run time
Install on Linux GCC	1m 53s
Clang tidy on ubuntu-22.04 (Debug)	4m 18s
Install on Linux Clang	1m 57s
Linux clang 18 C++20	4m 52s
Clang tidy on windows-latest (Debug)	5m 33s
Install on Windows MSVC	2m 34s
Linux clang 18	4m 36s
Clang tidy on ubuntu-22.04 (Release)	4m 17s
Linux clang 17	4m 12s
Clang tidy on windows-latest (Release)	5m 42s
Linux clang 16	5m 10s
Intel LLVM-based compiler	1m 50s
Linux clang 15	4m 52s
Linux clang 18 libc++	2m 39s
Linux clang 14	4m 52s
Windows VS v143	5m 38s
Linux clang 17 libc++	2m 40s
Linux clang 13	4m 43s
Windows VS ClangCL	11m 10s
Linux clang 16 libc++	5m 28s
Linux clang 12	5m 12s
Linux clang 15 libc++	2m 39s
Linux clang 11	5m 10s
Linux clang 14 libc++	2m 54s
Linux clang 10	5m 26s
Linux clang 13 libc++	2m 44s
Linux clang 9	5m 5s
Linux clang 12 libc++	2m 42s
Linux gcc 14 C++20	6m 13s
Linux clang 11 libc++	2m 55s
Linux gcc 14	5m 8s
Linux clang 10 libc++	2m 55s
Linux gcc 13	8m 13s
Linux clang 9 libc++	2m 46s
Linux gcc 12	11m 26s
Linux gcc 11	9m 4s
	2h 49m 28s

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