[CIR][CIRGen][LowerToLLVM] Support address space casting

[seven-mile]

• New CastKind::addrspace_cast for cir.cast
• TargetCIRGenInfo::performAddrSpaceCast helper for non-constant values only
• CIRGen for address space casting of pointers and references
• Lowering to LLVM

[seven-mile]

Note that other versions of TargetCIRGenInfo::performAddrSpaceCast should include GlobalOp and ConstOp, which are unified as llvm::Constant in LLVM IR. Should track it by another issue.

[seven-mile]

Rebased. Updated except ABI-related nullptr representation.

[seven-mile]

Updated as suggested.