Fix cycle error when emitting suggestion for mismatched `fn` type
Fixes #66667
Previously, we called `tcx.typeck_tables_of` when determining whether or
not to emit a suggestion for a type error. However, we might already be
type-checking the `DefId` we pass to `typeck_tables_of` (it could be
anywhere in the query stack).
Fortunately, we only need the function signature, not the entire
`TypeckTables`. By using `tcx.fn_sig`, we avoid the possibility of cycle
errors while retaining the ability to emit a suggestion.