[AST] Only dump desugared type when visibly different (#65214)

These are an artifact of how types are structured but serve little
purpose, merely showing that the type is sugared in some way. For
example, ElaboratedType's existence means struct S gets printed as
'struct S':'struct S' in the AST, which is unnecessary visual clutter.
Note that skipping the second print when the types have the same string
matches what we do for diagnostics, where the aka will be skipped.