Bump third_party/imgui/repo from `3816d47` to `313676d` #4811

Job	Run time
Build on Linux (gcc_debug)	58m 28s
Build on Darwin (clang, python_lib, simulated)	59m 51s
Build on Linux (fake, gcc_release, clang, simulated)	2h 26m 15s
Build on Linux (python_lib)	38m 56s
Build on Linux (coverage)	15m 16s
	5h 18m 46s

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