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