[mlir][spirv] Consolidate std.constant to spv.constant conversions

This commit merges the DRR pattern for std.constant to spv.constant
conversion into the C++ OpConversionPattern. This allows us to have
remove the DRR pattern file. Along the way, this commit enhanced
std.constant to spv.constant conversion to consider type conversions,
which means converting the underlying attributes if necessary.

Differential Revision: https://reviews.llvm.org/D76246