Add Vulkan validation rules for BuiltIn variables

Added a framework for validation of BuiltIn variables. The framework
allows implementation of flexible abstract rules which are required for
built-ins as the information (decoration, definition, reference) is not
in one place, but is scattered all over the module.

Validation rules are implemented as a map
id -> list<functor(instrution)>

Ids which are dependent on built-in types or objects receive a task
list, such as "this id cannot be referenced from function which is
called from entry point with execution model X; propagate this rule
to your descendants in the global scope".

Also refactored test/val/val_fixtures.

All built-ins covered by tests