[libFuzzer] Update InputInfo.TimeOfUnit when replacing it in the corpus.

Previously, when the fuzzing loop replaced an input in the corpus, it didn't update the execution time of the input. Therefore, some schedulers (e.g. Entropic) would adjust weights based on the incorrect execution time.

This patch updates the execution time of the input when replacing it.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D111479