C api for setting spec constants

There are functions that depend on the values of specialization constants (for example, get_declared_struct_member_size). There is no way to get the correct result in the C API because there is no API for setting specialization constants.

Start at the C API entry point for get_declared_struct_member_size and trace how specialization-constant values reach reflection functions. Determine the setter's expected scope and behavior, then verify that dependent results use the supplied constants; done means the C API can set them and return correct values.

Written by the indexing model from the issue text.