Miscellaneous fixes to SPIR-V Deserializer (details below).

1) Process and ignore the following debug instructions: OpSource,
OpSourceContinued, OpSourceExtension, OpString, OpModuleProcessed.
2) While processing OpTypeInt instruction, ignore the signedness
specification. Currently MLIR doesnt make a distinction between signed
and unsigned integer types.
3) Process and ignore BufferBlock decoration (similar to Buffer
decoration). StructType needs to be enhanced to track this attribute
since its needed for proper validation checks.
4) Report better error for unhandled instruction during
deserialization.

PiperOrigin-RevId: 271057060