[libFuzzer] Fix MSan false positives with custom mutators.

We need to unpoison parameters before calling into MSan-instrumented
user-defined mutators.

Addresses https://github.com/google/oss-fuzz/issues/4605.

Reviewed By: metzman

Differential Revision: https://reviews.llvm.org/D100355

GitOrigin-RevId: 423024904825d147c69e8999537b6b813bc24999
1 file changed