[libFuzzer] Replace -seed_corpus to better support fork mode on Win
Pass seed corpus list in a file to get around argument length limits on Windows.
This limit was preventing many uses of fork mode on Windows.
Reviewers: kcc, morehouse
Reviewed By: kcc
Subscribers: #sanitizers, llvm-commits
Tags: #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D60980