[libFuzzer] Add feature to not use AFL's deferred forkserver.

A small but substantial minority of libFuzzer-based fuzzers run code that
does not play well with fork in global constructors or LLVMFuzzerInitialize.
This patch allows these fuzzers to use afl_driver by allowing them to
opt-out of using AFL's deferred forkserver which deferres calling fork until
after this code.

Patch By: metzman

Differential Revision: https://reviews.llvm.org/D45744

llvm-svn: 330652