[libFuzzer] Create single template for visiting Inline8bitCounters

Summary:
Created IterateInline8bitCounters, a single template for visiting  Inline8bitCounters (nested for loop)
Made InitializeUnstableCounters and UpdateUnstableCounters both send a lambda to IterateInline8bitCounters.

Patch by Kyungtak Woo (@kevinwkt).

Reviewers: Dor1s, metzman, kcc, morehouse

Reviewed By: metzman, morehouse

Subscribers: delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D49453

llvm-svn: 337403