[mlir][spirv] Support spec constants as GlobalVar initializer (#75660)

Changes include:

- spirv serialization and deserialization needs handling in cases when
GlobalVariableOp initializer is defined using spirv SpecConstant or
SpecConstantComposite op, currently even though it allows SpecConst, it
only looked up in for GlobalVariable Map to find initializer symbol
reference, change is fixing this and extending the support to
SpecConstantComposite as an initializer.
- Adds tests to make sure GlobalVariable can be initialized using
specialized constants.

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Co-authored-by: Lei Zhang <antiagainst@gmail.com>