Correctly use UTF-8 indices when highlighting diagnostic source

[arthurcro]

Bug/issue #, if applicable: #678

Summary

Previously, the default diagnostic formatter would crash due to incorrect indices handling when trying to highlight a diagnostic source range.

A diagnostic's SourceRange is the range in the source file that emits the diagnostic. When highlighting a diagnostic source range, we use the column property of the upperBound and lowerBound to find indices of a source string that should be highlighted.

The column property is The number of bytes in UTF-8 encoding.

/// The number of bytes in UTF-8 encoding from the start of the line to the character at this source location.

However, this was not taken into account previously when calculating the indices.

This pull request fixes this crash by using proper UTF-8 indices.

Testing

This functionally can be tested using the steps mentioned in #678.

1. Clone https://github.com/apple/swift.git
2. Install a recent development snapshot of Swift
3. Run DocC from this snapshot this way (assuming swift is the clone from step 1):
   docc preview --allow-arbitrary-catalog-directories swift/docs

Alternatively, the issue is easily reproducible by adding an emoji to a source line that would emit a diagnostic.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

@swift-ci please test