[6.0][Macros] Don't allow macros to add accessors to let variables.

[hborla]

• Explanation: let declarations cannot be computed properties, and accessor macros are currently allowed to bypass this rule. This was accidental, so this change diagnoses expanded accessors on a let variable as an invalid macro expansion.
• Scope: Only impacts accessor macros attached to let variables
• Issue: rdar://122690037, [AccessorMacro] 'let' declarations cannot be computed properties swift-syntax#2491
• Risk: Low; the diagnostic is staged in as a warning until Swift 6.
• Testing: Added a new unit test.
• Reviewer: TBD
• Main branch PR: [Macros] Don't allow macros to add accessors to let variables. #73878

[hborla]

@swift-ci please test