[llvm] Add initial import of LLVM modules to mlir-translate

This adds an importer from LLVM IR or bitcode to the LLVM dialect. The importer is registered with mlir-translate.

Known issues exposed by this patch but not yet fixed:
  * Globals' initializers are attributes, which makes it impossible to represent a ConstantExpr. This will be fixed in a followup.
  * icmp returns i32 rather than i1.
  * select and a couple of other instructions aren't implemented.
  * llvm.cond_br takes its successors in a weird order.

The testing here is known to be non-exhaustive.

I'd appreciate feedback on where this functionality should live. It looks like the translator *from MLIR to LLVM* lives in Target/, but the SPIR-V deserializer lives in Dialect/ which is why I've put this here too.

PiperOrigin-RevId: 278711683