[libFuzzer] Extend the fuzz target intarface to allow -1 return value.

With this change, fuzz targets may choose to return -1
to indicate that the input should not be added to the corpus
regardless of the coverage it generated.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D128749

GitOrigin-RevId: 92fb310151d2b1e349695fc0f1c5d5d50afb3b52
2 files changed