[CIR][CodeGen][NFC] Refactor performAddrSpaceCast to consume CIR address space attributes

[seven-mile]

Originally TargetCodeGenInfo::performAddrSpaceCast consumes two unused parameters typed clang::LangAS. This PR changes its type to mlir::cir::AddressSpaceAttr to better fit the need of CIRGen.

In D32248: CodeGen: Cast alloca to expected address space, the author explained why these AS parameters are not used:

This is just the default implementation. The idea is that targets that need to do something more complex on a particular conversion — e.g. to make sure that null pointers are translated correctly when they have different bit-patterns — can easily do so.

Further more, I'm confident that the CIR AS is also capable of providing custom behaviors like above for those targets.

[seven-mile]

Motive

In order to fix alloca AS in #682 in a way that works seamlessly with CIR's unified address space, we intend to change clang::LangAS getASTAllocaAddressSpace() to mlir::cir::AddressSpaceAttr getCIRAllocaAddressSpace().

Apart from that, we have several addrspace casts to fix. For example (in OG CodeGen):

- LangAS AAS = getASTAllocaAddressSpace();
+ mlir::cir::AddressSpaceAttr AAS = getCIRAllocaAddressSpace();
  
- LangAS EAS = E->getType()->getPointeeType().getAddressSpace();
+ mlir::cir::AddressSpaceAttr EAS = builder.getAddrSpaceAttr(
+      E->getType()->getPointeeType().getAddressSpace());
  if (AAS != EAS) {
    // ...
    return RValue::get(getTargetHooks().performAddrSpaceCast(*this, AI, AAS,
                                                              EAS, Ty));
  }

In this code snippets, AAS has type LangAS and therefore fits the need of performAddrSpaceCast. But then we want to change its type to mlir::cir::AddressSpaceAttr. It's easy to get CIR AS from LangAS, but not vice versa. So I think this refactor provides cleaner API and do no harm to future development.

[jopperm]

Seems like the logical next step to me.

[bcardosolopes]

Thanks a bunch for the more incremental changes!