[MLIR][SPIRV] ModuleCombiner: deduplicate global vars, spec consts, and funcs.

This commit extends the functionality of the SPIR-V module combiner
library by adding new deduplication capabilities. In particular,
implementation of deduplication of global variables and specialization
constants, and functions is introduced.

For global variables, 2 variables are considered duplicate if they either
have the same descriptor set + binding or the same built_in attribute.

For specialization constants, 2 spec constants are considered duplicate if
they have the same spec_id attribute.

2 functions are deduplicated if they are identical. 2 functions are
identical if they have the same prototype, attributes, and body.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D90951