layers: Fix extension func/core struct codegen

Fix code generation for extension functions receiving struct parameter
types defined by core Vulkan.  Extensions are processed as separate
features by the code generator, and the type info required for structure
generation was not being shared across features.  The code generator has
been modified to share type info across features so that the validation
code generated for extension functions includes validation for core
structures:
 - Prevent unique objects and parameter validation code generators from
   clearing struct type info data structures at the start of feature
   processing.
 - Remove unused data structures from unique objects code generator.
 - Adds handle unwrapping and parameter validation for elements in the
   vkCmdPushDescriptorSetKHR pDescriptorWrites parameter.
 - Adds handle unwrapping and parameter validation for elements in the
   vkCreateSharedSwapCHainsKHR pCreateInfos parameter.
 - Adds VkAllocationCallback parameter validation to the WSI and
   descriptor update template extensions functions.

Change-Id: I016aa6550681dbf7d6bda834272374ce63ed1940