[5.9][Macros] Add missing macro validation.

[hborla]

• Explanation: Fixes the last remaining holes (that I know of!) in macro validation, adding diagnostics for:
  • Extension macros generating names not documented in the @attached(extension) attribute.
  • Extension macros generating conformances not documented in the @attached(extension) attribute.
  • Macro attributes attached to declarations that the macro does not apply to, such as a member macro attached to an import declaration.
• Scope: Only affects attached macros.
• Risk: Low due to narrow scope.
• Testing: Added new test cases.
• Reviewer: @DougGregor
• Issue: rdar://111685434&112163965
• Main branch PR: [Macros] Add missing macro validation. #67106, [Macros] Diagnose undocumented conformances in extension macro expansions. #67383

[hborla]

@swift-ci please test