Bump third_party/imgui/repo from `3c435c0` to `ccb6646` #5547

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	59m 7s
Build on Linux (gcc_debug)	1h 4m 42s
Build on Darwin (clang, simulated)	45m 28s
Build on Linux (coverage)	14m 48s
Build on Linux (python_lib)	23m 27s
Build on Linux (python lighting-app)	5m 46s
	3h 33m 18s

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