[5.6] Throw exception for has() with MorphTo relationship

[staudenmeir]

It's a long-known issue that has() and whereHas() don't support MorphTo relationships (#5429, #18523). There are workarounds, but the fundamental problem is basically unfixable.

Depending on the actual code, the user gets an exception (e.g. "unknown column") or the query "successfully" executes pointless SQL:

Comment::has('commentable')->get();

select *
from "comments"
where exists (
  select *
  from "comments" as "laravel_reserved_0"
  where "laravel_reserved_0"."id" = "laravel_reserved_0"."commentable_id"
)

With this PR, the user at least receives a proper exception.
Should we include a link to one of the GitHub issues (for the workarounds)?