[llvm-dwarfdump][Statistics] Don't count coverage less than 1% as 0%

Summary:
This is a follow up for D70548.
Currently, variables with debug info coverage between 0% and 1% are put into
zero-bucket. D70548 changed the way statistics calculate a variable's coverage:
we began to use enclosing scope rather than a possible variable life range.
Thus more variables might be moved to zero-bucket despite they have some debug
info coverage.
The patch is to distinguish between a variable that has location info but
it's significantly less than its enclosing scope and a variable that doesn't
have it at all.

Reviewers: djtodoro, aprantl, dblaikie, avl

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71070