[compiler-rt] Move FDP to include/fuzzer/FuzzedDataProvider.h for easier use.

Summary:
FuzzedDataProvider is a helper class for writing fuzz targets that fuzz
multple inputs simultaneously. The header is supposed to be used for fuzzing
engine agnostic fuzz targets (i.e. the same target can be used with libFuzzer,
AFL, honggfuzz, and other engines). The common thing though is that fuzz targets
are typically compiled with clang, as it provides all sanitizers as well as
different coverage instrumentation modes. Therefore, making this FDP class a
part of the compiler-rt installation package would make it easier to develop
and distribute fuzz targets across different projects, build systems, etc.
Some context also available in https://github.com/google/oss-fuzz/pull/2547.

This CL does not delete the header from `lib/fuzzer/utils` directory in order to
provide the downstream users some time for a smooth migration to the new
header location.

Reviewers: kcc, morehouse

Reviewed By: morehouse

Subscribers: lebedev.ri, kubamracek, dberris, mgorny, delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D65661

llvm-svn: 367917