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

This commit adds initial support for SPIR-V OpSpecConstantOp
instruction. The following is introdcued:

- A new `spv.specConstantOperation` operation consisting of a single
region and of 2 operations within that regions (more details in the
docs of the op itself).
- A new `spv.yield` instruction that acts a terminator for
`spv.specConstantOperation`.

For now, the generic form of the new op is supported (i.e. no custom
parsing or printing). This will be done in a follow up patch.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D92232