`missing_const_for_fn`: consider constness of instance (#14759)

When determining when a function or method can be called from a `const`
context, the determination must be made on the instance, not on the
declaration. This makes a difference, for example, with `const_trait`
traits whose implementations may or may not be `const`.

changelog: [`missing_const_for_fn`]: when checking if a function or
method can be called from a `const` context, look at the concrete
implementation rather than at the trait definition

Fixes rust-lang/rust-clippy#14658

r? @Jarcho

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