[libFuzzer] When printing NEW_FUNC, use 1-base indexing.

Summary: Otherwise we print things like [0/1] which is visually confusing.

Reviewers: kcc

Reviewed By: kcc

Subscribers: delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D47837

llvm-svn: 334234