Auto merge of #5889 - ebroto:5886_or_fun_call_const_0_args, r=Manishearth

Auto merge of #5889 - ebroto:5886_or_fun_call_const_0_args, r=Manishearth

Avoid or_fun_call for const_fn with no args

Based on #5682 by @lzutao

This avoids a subset of false positives, specifically those related to `const fn`s that take no arguments.
For the rest, a much more involved fix would be needed, see https://github.com/rust-lang/rust-clippy/pull/5682#issuecomment-638681210.

So this does *not* solve #5658

changelog: Avoid triggering [`or_fun_call`] with `const fn`s that take no arguments.

Fixes #5886