Bump third_party/imgui/repo from `3816d47` to `313676d` #4812

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 18m 5s
Build on Linux (fake, gcc_release, clang, simulated)	2h 12m 13s
Build on Linux (coverage)	20m 16s
Build on Linux (gcc_debug)	1h 6m 44s
Build on Linux (python_lib)	38m 59s
	5h 36m 17s

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