[llvm-cov] Avoid over-counting covered lines and regions

[llvm-cov] Avoid over-counting covered lines and regions

* Fix an unsigned integer overflow in the logic that computes the
  number of uncovered lines in a function.

* When aggregating region and line coverage summaries, take into account
  that different instantiations may have a different number of regions.

The new test case provides test coverage for both bugs. I also verified
this change by preparing a coverage report for a stage2 build of llc --
the new assertions should detect any outstanding over-counting bugs.

Fixes PR34613.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313417 91177308-0d34-0410-b5e6-96231b3b80d8