[5.5][Diagnostics] Correct the insert location of missing return type fix-it.

[hborla]

Cherry-pick of #38112

• Explanation: In 5.5, a new fix-it was added to automatically insert a return type placeholder for a function that mistakenly returns a non-Void result from a Void function. This fix-it is always inserted after the parameter list, which isn't correct for functions with effects like throws and async. This change moves the fix-it location to after the effects if they exist.
• Scope: This change only affects invalid code that returns a non-Void result from a Void function.
• Risk: Very low.
• Testing: Added a handful of diagnostics tests that check the fix-it location.
• Reviewer: @xedin @theblixguy

Resolves: rdar://79345188

[hborla]

@swift-ci please test

[hborla]

@swift-ci please test Windows platform