docs: Add some information about Fuzzing LLVM itself

This splits some content out of the libFuzzer docs and adds a fair
amount of detail about the fuzzers in LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315544 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed