Bump third_party/imgui/repo from `3c435c0` to `5b7feeb` #5565

Job	Run time
Build on Darwin (clang, simulated)	54m 3s
Build on Linux (fake, gcc_release, clang, simulated)	54m 19s
Build on Linux (coverage)	14m 36s
Build on Linux (gcc_debug)	1h 4m 12s
Build on Linux (python_lib)	23m 34s
Build on Linux (python lighting-app)	6m 47s
	3h 37m 31s

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