fix a false positive in two `interior_mutable_const` lints where a constant with enums gets linted
even if it uses a clearly unfrozen variant. Note that the code uses the MIR interpreter, which
the author of #3962 thought unlikely to be a solution. This might be over-engineering;
but, I think it's important to be able to work with the 'http' crate (#3825).
* remove a 'ERROR' comment from `borrow`
`Vec<AtomicUsize>` itself is `Freeze` as it holds the atomic in heap
* remove `ONCE_INIT` from `declare`
it seems like an artifact from previous spliting