Fix unused_parens false positive

[benschulz]

Resolves #143653.

The "no bounds exception" was indiscriminately set to OneBound for referents and pointees. However, if the reference or pointer type itself appears in no-bounds position, any constraints it has must be propagated.

// unused parens: not in no-bounds position
fn foo(_: Box<(dyn Send)>) {}

// unused parens: in no-bounds position, but one-bound exception applies
fn bar(_: Box<dyn Fn(&u32) -> &(dyn Send)>) {} 

// *NOT* unused parens: in no-bounds position, but no exceptions to be made
fn baz(_: Box<dyn Fn(&u32) -> &(dyn Send) + Send>) {}

[rustbot]

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[compiler-errors]

How does this behave with dyn FnMut(&()) -> &(dyn Trait) (i.e. one bound that is parenthesized)?

[benschulz]

I've added that case to the test: We continue to emit a warning.