[WIP] Update SwiftBuild progress reporting

[bripeticca]

Currently a WIP.

Addresses:

• Comments under Capture code snippet from diagnostic compiler output #9362 related to testing
• Removes the stepping fraction from being printed when dealing with events that report a negative percentage; the idea here is that if the progress is in the negative, this seems to be a pre-build step where there is no real benefit to reporting the "percentage complete" here.

TODO:

• Some event messages related to progress updates seem to have a stepping fraction embedded into their suffix; will have to determine whether we want to use these instead of the ninja-style progress reporting or if we should parse this out and send this to the ninja progress animation itself to handle.