[compiler-rt] Rename FuzzedDataProvider.h to .hpp and other minor changes.

Summary:
.hpp makes more sense for this header as it's C++ only, plus it
contains the actual implementation.

Reviewers: Dor1s

Reviewed By: Dor1s

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

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D65812

llvm-svn: 368054