Materialize spv.constants at use sites

In SPIR-V binary format, constants are placed at the module level
and referenced by instructions inside functions using their result
<id>s. To model this natively (using SSA values for result <id>s),
it means we need to have implicit capturing functions. We will
lose the ability to have function passes if going down that path.

Instead, this CL changes to materialize constants at their use
sites in deserialization. It's cheap to copy constants in MLIR
given that attributes is uniqued to MLIRContext. By localizing
constants into functions, we can preserve isolated functions.

PiperOrigin-RevId: 264582532