Initial version of storing SourceElements in descriptors

Introduce SourceElement, JavaSourceElementFactory, DeclarationDescriptorWithSource
Implement getSource() for eager, lazy and java descriptors
REVIEW: minor
Rewrite BindingContextUtils.descriptorToDeclaration utilities family to use SourceElement for getting PSI

Remove DESCRIPTOR_TO_DECLARATION context key
Refactor: Remove context parameter from BindingContextUtils.descriptorToDeclaration utilities family