[Refactoring] Avoid redeclarations or shadowing in async refactored code

When converting a call or function, rename declarations such that
redeclaration errors and shadowing are avoided. In some cases this will
be overly conservative, but since any renamed variable can be fixed with
edit all in scope, this is preferred over causing redeclaration errors
or possible shadowing.

Resolves rdar://73973517

Author: bnbarham

include/swift/AST/DiagnosticsRefactoring.def

@@ -62,8 +62,6 @@ ERROR(unknown_callback_conditions, none, "cannot refactor complex if conditions"
 
 ERROR(mixed_callback_conditions, none, "cannot refactor mixed nil and not-nil conditions", ())
 
-ERROR(callback_multiple_bound_names, none, "cannot refactor when multiple names bound to single declaration, had '%0' and found '%1'", (StringRef, StringRef))
-
 ERROR(callback_with_fallthrough, none, "cannot refactor switch with fallthrough", ())
 
 ERROR(callback_with_default, none, "cannot refactor switch with default case", ())