[MLIR][SPIRVToLLVM] Conversion of SPIR-V struct type without offset

This patch introduces type conversion for SPIR-V structs. Since
handling offset case requires thorough testing, it was left out
for now. Hence, only structs with no offset are currently
supported. Also, structs containing member decorations cannot
be translated.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D83403