Misc changes to lowering to SPIR-V.

These changes to SPIR-V lowering while adding support for lowering
SUbViewOp, but are not directly related.
- Change the lowering of MemRefType to
  !spv.ptr<!spv.struct<!spv.array<...>[offset]>, ..>
  This is consistent with the Vulkan spec.
- To enable testing a simple pattern of lowering functions is added to
  ConvertStandardToSPIRVPass. This is just used to convert the type of
  the arguments of the function. The added function lowering itself is
  not meant to be the way functions are eventually lowered into SPIR-V
  dialect.

PiperOrigin-RevId: 282589644