[clang][SPIR][SPIRV] Don't generate constant NULL from addrspacecast generic NULL (#165353)

Fix a regression caused by 1ffff05.
OpenCL/SPIRV generic address space doesn't cover constant address space.

---------

Co-authored-by: Alexey Bader <[email protected]>

Author: wenju-he

clang/lib/CodeGen/Targets/SPIR.cpp

@@ -260,7 +260,8 @@ CommonSPIRTargetCodeGenInfo::getNullPointer(const CodeGen::CodeGenModule &CGM,
   LangAS AS = QT->getUnqualifiedDesugaredType()->isNullPtrType()
                   ? LangAS::Default
                   : QT->getPointeeType().getAddressSpace();
-  if (AS == LangAS::Default || AS == LangAS::opencl_generic)
+  if (AS == LangAS::Default || AS == LangAS::opencl_generic ||
+      AS == LangAS::opencl_constant)
     return llvm::ConstantPointerNull::get(PT);
 
   auto &Ctx = CGM.getContext();