Fix layout checks for StorageBuffer and PushConstant storage classes

Fixes #1664 :  PushConstant with Block follows storage buffer rules
  PushConstant variables were being checked with block rules, which are
  too strict.
Fixes #1606 :  StorageBuffer with Block layout follows buffer rules
  StorageBuffer variables were not being checked before.

Fix layout messages: say storage class and decoration
  We need to provide more information about storage class and decoration.