[libFuzzer] Disable len_control by default if LLVMFuzzerCustomMutator is used.
Some custom mutators may not peform well when size restriction is
enforced by len_control. Because of that, it's safer to disable len_control
by default in such cases, but still allow users to enable it manually.
Bug example: https://bugs.chromium.org/p/chromium/issues/detail?id=919530.
Tested manually with LPM-based and regular fuzz targets.
Reviewers: kcc, vitalybuka, metzman
Reviewed By: kcc, metzman
Subscribers: delcypher, #sanitizers, llvm-commits
Tags: #llvm, #sanitizers
Differential Revision: https://reviews.llvm.org/D63334