[spv-out] Let write_type_declaration_local handle all LocalTypes.

Change `write_type_declaration_local` to handle any type that can be represented
as a `LocalType`. This means that Image and Sampler types are handled there now.

Change `write_type_declaration_arena` to defer to `write_type_declaration_local`
to handle any type that can be represented as a `LocalType`. This makes the code
for those types in `write_type_declaration_arena` redundant, so remove it.

However, continue to request SPIR-V capabilities for image types from
`write_type_declaration_arena`, since that function is fallible, while
`write_type_declaration_local` is not.