[6.0] Avoid visiting symbol markup twice when there's a documentation extension file (#951)

[d-ronnqvist]

• Explanation: This addresses the source of duplicate diagnostics by not visiting a symbol's markup twice when a symbol has documentation from both an in-source doc comment and from a documentation extension file. This was a source of crashes before https://github.com/apple/swift-docc/pull/948
• Scope: Potential duplicated diagnostics.
• Issue: rdar://129677496
• Risk: Low
• Testing: New tests verify a single pass updates and produces diagnostics for both sources of documentation (in-source doc comment and a documentation extension file). Existing tests and manual testing verifies that no expected diagnostics are missing.
• Reviewer: @patshaughnessy
• Original PR: #951

[d-ronnqvist]

@swift-ci please test