Bump third_party/imgui/repo from `3c435c0` to `cb16568` #900

Job	Run time
REPL Tests - Linux	1h 2m 6s
Test Suites - Darwin (no-ble-asan-clang)	1h 30m 34s
Test Suites - Darwin (no-ble-tsan-clang)	1h 49m 43s
Test Suites - Linux (no-ble-tsan-clang)	1h 41m 7s
REPL Tests - Darwin	0s
	6h 3m 30s

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