[libFuzzer] Stop fuzzing if no new corpus were found within a specified period (#176177)

This patch adds a new command-line option `-stale_corpus_timeout=N`
which allows you to specify that the fuzzer should exit if it hasn't
found a new corpus within a certain amount of time. It also adds to
stats how many seconds a new path has not been found. An analog of
`AFL_EXIT_ON_TIME`.

GitOrigin-RevId: af2d76922b54fc347faac9727b0748387fea043f
6 files changed