[spirv] Add module level constants in SPIR-V dialect

[MaheshRavishankar]

Global variables in SPIR-V now use symbol names instead of returning a SSA values. The initializers of global variables also use these symbol names. Since constants can be used as initializers, module-level constants need to also have symbol names (instead of returning a symbol value). Also need a mechanism to either materialize these global constant within a function scope so that there is an SSA value to use with other instructions.