[llvm-cov] Remove a redundant field. NFC.

[llvm-cov] Remove a redundant field. NFC.

The "NotCovered" fields in the region and line summary structs are
redundant. We should remove them to make the code clearer.

As a follow-up, the "NotCovered" entries should be removed from the
reports as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313415 91177308-0d34-0410-b5e6-96231b3b80d8