Since the cache is now more reliable, some workarounds and optimizations
in the instantiation process are not needed anymore.

Also avoid instantiation of base classes when expandTemplates is
disabled.

As a bonus, we now resolve decltype of template function for a type that is not
referenced anywhere but in the decltype.

Change-Id: Idf42ba7280992db477c9aa62bb1815b27174594d
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>