Bump third_party/imgui/repo from `3c435c0` to `9bd5d8a` #2356

Job	Run time
Build on Darwin (clang, python_lib, simulated)	2h 31m 24s
Build on Linux (fake, gcc_release, clang, simulated)	2h 0m 14s
Build on Linux (coverage)	13m 23s
Build on Linux (gcc_debug)	1h 8m 12s
Build on Linux (python_lib)	21m 54s
Build on Linux (python lighting-app)	6m 30s
	6h 21m 37s

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