Implement WebGPU->Vulkan initializer conversion for 'Function' variables (#2513)

WebGPU requires certain variables to be initialized, whereas there are
known issues with using initializers in Vulkan. This PR is the first
of three implementing a pass to decompose initialized variables into
a variable declaration followed by a store. This has been broken up
into multiple PRs, because there 3 distinct cases that need to be
handled, which require separate implementations.

This first PR implements the basic infrastructure that is needed, and
handling of Function storage class variables. Private and Output will
be handled in future PRs.

This is part of resolving #2388