fix($compile): throw error when requestng new and isolate scopes (async)

While directives are not allowed to request both a new (normal) and an
isolate scope on the same element, the relevant check was not performed
correctly when the directive requesting the isolate scope had a lower
priority and specified a `templateUrl`.
In that case, the check was deferred until the template was fetched, but
the info about other directives requesting a new (normal) scope was not
properly preserved (in `previousCompileContext`).

This commit fixes this, so now an error is thrown (as expected).

Fixes #12215
Closes #12217