[Typechecker] Emit a specialised diagnostic for redeclaration errors when the declaration is synthesised

[theblixguy]

At the moment, some redeclaration errors involving synthesised declarations can cause compiler crashes. For example:

@propertyWrapper 
struct Wrapper {
  var wrappedValue: Int
}

struct S {
  @Wrapper var foo1 = 123
  var _foo1 = 123
}

This currently crashes in the AST verifier on master and in SILGen on 5.3. Upon investigation, this seems to be a regression caused by #31037.

To fix the problem, I have added a specialised diagnostic for situations where the current declaration is synthesised. This also fixes a problem where the redeclaration diagnostics did not have a valid source location.

Resolves SR-12839
Resolves rdar://problem/63496795

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test

[hborla]

Looks great, thank you @theblixguy !

[theblixguy]

@hborla Do you know if this can be cherry-picked to 5.3? I'd like to do that so we can fix the crash, alternatively we can revert #31037 on the 5.3 branch only (but that means we don't get the improved diagnostics).

[hborla]

@theblixguy yes, please prepare a PR for the 5.3 branch. Thank you!