vulkan/enum_to_str: Handle out-of-order aliases

vulkan/enum_to_str: Handle out-of-order aliases

The current code can only handle enum aliases if the original enum is
declared first followed by the alias as we walk the XML in a linear
fashion.  This commit allows us to handle aliases where the alias
declaration comes before the thing it's aliasing.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>