[MLIR][SPIRV] Add initial support for OpSpecConstantComposite.

This commit adds support to SPIR-V's composite specialization constants.
These are specialization constants which are composed of other spec
constants (whehter scalar or composite), regular constatns, or undef
values.

This commit adds support for parsing, printing, verification, and
(De)serialization.

A few TODOs are still in order:
- Supporting more types of constituents; currently, only scalar spec constatns are supported.
- Extending `spv._reference_of` to support composite spec constatns.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D88568