[clang][NFC] Tighten up code for GetGlobalVarAddressSpace

[wingo]

Summary

The LangAS local is only used in the OpenCL case; move its decl
inwards.

[wingo]

Just a drive-by cleanup while poking other address-space-related things.

[Anastasia]

LGTM! Thanks

clang/lib/CodeGen/CodeGenModule.cpp

 }
 
 CharUnits CodeGenModule::GetTargetTypeStoreSize(llvm::Type *Ty) const {
   return Context.toCharUnitsFromBits(
       getDataLayout().getTypeStoreSizeInBits(Ty));
 }
 
 LangAS CodeGenModule::GetGlobalVarAddressSpace(const VarDecl *D) {
-  LangAS AddrSpace = LangAS::Default;
   if (LangOpts.OpenCL) {
-    AddrSpace = D ? D->getType().getAddressSpace() : LangAS::opencl_global;
-    assert(AddrSpace == LangAS::opencl_global ||
-           AddrSpace == LangAS::opencl_global_device ||
-           AddrSpace == LangAS::opencl_global_host ||
-           AddrSpace == LangAS::opencl_constant ||
-           AddrSpace == LangAS::opencl_local ||
-           AddrSpace >= LangAS::FirstTargetAddressSpace);
-    return AddrSpace;
+    LangAS AS = D ? D->getType().getAddressSpace() : LangAS::opencl_global;
+    assert(AS == LangAS::opencl_global ||
+           AS == LangAS::opencl_global_device ||
+           AS == LangAS::opencl_global_host ||
+           AS == LangAS::opencl_constant ||
+           AS == LangAS::opencl_local ||
+           AS >= LangAS::FirstTargetAddressSpace);
+    return AS;
   }
 
   if (LangOpts.SYCLIsDevice &&
       (!D || D->getType().getAddressSpace() == LangAS::Default))
     return LangAS::sycl_global;
 
   if (LangOpts.CUDA && LangOpts.CUDAIsDevice) {
     if (D && D->hasAttr<CUDAConstantAttr>())