Issue Description

#6174 made progress on the issue with tasks understanding WarningsAsErrors, but there's still work to do.

Steps to Reproduce

Create a task that returns !Log.HasLoggedErrors
Set up your build such that:

1. All warnings will be treated as errors.
2. Add a different warning to be treated as a message.

Expected Behavior

Task returns true.

Actual Behavior

Task will return false.

Analysis

See my comment in this PR: #6304.

Because of the implied rule that a non-null empty set of WarningsAsErrors means that all warnings are treated as errors, there's no way to have a single set that represents whether or not any given warning should be treated as an error.