[CIR][Builder][NFC] Refactor getPointerTo considering AddressSpaceAttr (#746)

After switching to a separate address space attribute in CIR, we need
better helper methods in CIR builder.

This PR provides:
* two versions of `getPointerTo`, consuming `LangAS` or
`AddressSpaceAttr`.
* an extra helper method `getAddrSpaceAttr(LangAS)` that is used by
`getPointerTo(LangAS)` and other potential use cases (mainly about
address space cast in the future).

Calls to `getPointerTo` without addrspace will invoke the version
consuming `AddressSpaceAttr` and return a type with null addrspace
attribute, which should be exactly what we expect for CIRBuilder.

Author: seven-mile

clang/include/clang/CIR/Dialect/Builder/CIRBaseBuilder.h

@@ -75,13 +75,19 @@ class CIRBaseBuilderTy : public mlir::OpBuilder {
     return mlir::cir::IntType::get(getContext(), N, true);
   }
 
-  mlir::cir::PointerType
-  getPointerTo(mlir::Type ty, clang::LangAS langAS = clang::LangAS::Default) {
-    mlir::cir::AddressSpaceAttr addrSpaceAttr;
-    if (langAS != clang::LangAS::Default)
-      addrSpaceAttr = mlir::cir::AddressSpaceAttr::get(getContext(), langAS);
+  mlir::cir::AddressSpaceAttr getAddrSpaceAttr(clang::LangAS langAS) {
+    if (langAS == clang::LangAS::Default)
+      return {};
+    return mlir::cir::AddressSpaceAttr::get(getContext(), langAS);
+  }
+
+  mlir::cir::PointerType getPointerTo(mlir::Type ty,
+                                      mlir::cir::AddressSpaceAttr cirAS = {}) {
+    return mlir::cir::PointerType::get(getContext(), ty, cirAS);
+  }
 
-    return mlir::cir::PointerType::get(getContext(), ty, addrSpaceAttr);
+  mlir::cir::PointerType getPointerTo(mlir::Type ty, clang::LangAS langAS) {
+    return getPointerTo(ty, getAddrSpaceAttr(langAS));
   }
 
   mlir::cir::PointerType