[libfuzzer] Refactoring coverage state-management code.

It is now less state-dependent and will allow easier comparing of
coverages of different units.

Differential Revision: http://reviews.llvm.org/D20085

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269140 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed