[libFuzzer] Expose fuzzer::FuzzerDriver()

Summary:
Allows other-language libFuzzer derivatives to invoke the driver.

Patch By: Ian Eldred Pudney <ipudney@google.com>

Reviewers: morehouse, kcc

Reviewed By: morehouse

Subscribers: #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D84425