[6.0] Handle various out-of-bounds diagnostic ranges in default diagnostics formatter (#947)

[d-ronnqvist]

• Explanation: This adds additional input validation in the default diagnostic formatter, instead of trusting that the diagnostic source range is correct.
• Scope: Potential crashes for diagnostics that have out-of-bounds source ranges due to other bugs.
• Issue: rdar://129586253 #729
• Risk: Low
• Testing: New tests verify that incorrectly created diagnostics with out-of-bounds source ranges are handled by the default diagnostic formatter.
• Reviewer: @Kyle-Ye
• Original PR: #947

[d-ronnqvist]

@swift-ci please test