[compiler-rt] Refactor FuzzedDataProvider for better readability.

Summary: Separate class definition and actual methods implementation. The main
goal is to keep the list of available methods in a concise readable form inside
the class definition.

Reviewers: hctim, metzman

Subscribers: dberris, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D76651