Bump third_party/imgui/repo from `3c435c0` to `f5f11e9` #5582

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	53m 15s
Build on Linux (gcc_debug)	1h 0m 57s
Build on Linux (python_lib)	21m 38s
Build on Darwin (clang, simulated)	42m 34s
Build on Linux (coverage)	14m 33s
Build on Linux (python lighting-app)	5m 50s
	3h 18m 47s

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