[libFuzzer] Add INFO output when LLVMFuzzerCustomMutator is found.

Summary:
there is an ongoing work on interchangeable custom mutators
(https://github.com/google/clusterfuzz/pull/1333/files#r367706283)
and having some sort of signalling from libFuzzer that it has loaded
a custom mutator would be helpful.

The initial idea was to make the mutator to print something, but given
the anticipated variety of different mutators, it does not seem possible
to make all of them print the same message to signal their execution.

Reviewers: kcc, metzman

Reviewed By: metzman

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D73136