[MLIR][SPIRV] Start module combiner.

This commit adds a new library that merges/combines a number of spv
modules into a combined one. The library has a single entry point:
combine(...).

To combine a number of MLIR spv modules, we move all the module-level ops
from all the input modules into one big combined module. To that end, the
combination process can proceed in 2 phases:

  (1) resolving conflicts between pairs of ops from different modules
  (2) deduplicate equivalent ops/sub-ops in the merged module. (TODO)

This patch implements only the first phase.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D90477