[MLIR][SPIRVToLLVM] Conversion for global and addressof

Inital conversion of `spv._address_of` and `spv.globalVariable`.
In SPIR-V, the global returns a pointer, whereas in LLVM dialect
the global holds an actual value. This difference is handled by
`spv._address_of` and `llvm.mlir.addressof`ops that both return
a pointer. Moreover, only current invocation is in conversion's
scope.

Reviewed By: antiagainst, mravishankar

Differential Revision: https://reviews.llvm.org/D84626