Make ExpressionVisitor::visitCall() clearer.

Rename checkForDecorators() to docstringTypeOverride(),
because it doesn't do anything involving decorators. (!)

Move the code choosing between class and function types to visitCall(),
because it's nothing to do with docstring contents and made the
different code paths very hard to follow.

Make it return the type it produces, rather than calling encounter()
itself, for clarity.

Use the new isAlias argument of functionForCalled instead of
some code to do the same thing.

Test init_class_no_decl is (still) XFAIL, we can't fix
that with the current system for class types.

Test call_class_no_decl worked before, but I broke that behaviour in
an earlier version of this patch and realised there wasn't a test.

Differential Revision: https://phabricator.kde.org/D3524