[6.2] use found disambiguation to calculate diagnostic range if present

[QuietMisdreavus]

• Explanation: Updates diagnostic solution ranges when a given disambiguation is insufficient to point to a single symbol.
• Scope: Prevents diagnostics from being given with wrong/invalid ranges, including negative column indices.
• Issues: rdar://150207195
• Original PRs: use found disambiguation to calculate diagnostic range if present #1213
• Risk: Low. The change is limited to diagnostic reporting code and shouldn't cause new errors in documentation processing. A new assertion is added but toolchain distributions of Swift-DocC turn assertions off so it shouldn't cause a problem for production uses.
• Testing: An automated test was added, and manual testing proved the change works with a larger set of triggering cases.
• Reviewers: @d-ronnqvist

[QuietMisdreavus]

@swift-ci Please test

[sofiaromorales]

@swift-ci please test