[MLIR][SPIRVToLLVM] Convert spv.constant scalars and vectors

This patch introduces conversion pattern for `spv.constant` with scalar
and vector types. There is a special case when the constant value is a
signed/unsigned integer (vector of integers). Since LLVM dialect does not
have signedness semantics, the types had to be converted to signless ints.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D82936