Instead of counting totally diagnostics, split the count into a count

Instead of counting totally diagnostics, split the count into a count
of errors and warnings.  This allows us to emit something like this:

2 warnings and 1 error generated.

instead of:

3 diagnostics generated.

This also stops counting 'notes' because they are just follow-on information
about the previous diag, not a diagnostic in themselves.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@100675 91177308-0d34-0410-b5e6-96231b3b80d8