Fix clone_constant_impl to correctly deal with null pointers

[aqjune]

Summary

This patch resolves llvm-c-test's following error

LLVM ERROR: LLVMGetValueKind returned incorrect type

which arises when the input bitcode contains a null pointer.

[jdoerfert]

LGTM

[aqjune]

I have no permission to land this patch - what is the best way for the next step?

test/Bindings/llvm-c/echo.ll

 @tl = thread_local global { i64, %S* } { i64 1, %S* @cst }
 @arr = linkonce_odr global [5 x i8] [ i8 2, i8 3, i8 5, i8 7, i8 11 ]
 @str = private unnamed_addr constant [13 x i8] c"hello world\0A\00"
 @locStr = private local_unnamed_addr constant [13 x i8] c"hello world\0A\00"
 @hidden = hidden global i32 7
 @protected = protected global i32 23
 @section = global i32 27, section ".custom"
 @align = global i32 31, align 4
+@nullptr = global i32* null
 
 @aliased1 = alias i32, i32* @var
 @aliased2 = internal alias i32, i32* @var
 @aliased3 = external alias i32, i32* @var
 @aliased4 = weak alias i32, i32* @var
 @aliased5 = weak_odr alias i32, i32* @var
 
 @ifunc = ifunc i32 (i32), i64 ()* @ifunc_resolver

tools/llvm-c-test/echo.cpp

if (LLVMIsAConstantStruct(Cst)) {
     for (unsigned i = 0; i < EltCount; i++)
       Elts.push_back(clone_constant(LLVMGetOperand(Cst, i), M));
     if (LLVMGetStructName(Ty))
       return LLVMConstNamedStruct(Ty, Elts.data(), EltCount);
     return LLVMConstStructInContext(LLVMGetModuleContext(M), Elts.data(),
                                     EltCount, LLVMIsPackedStruct(Ty));
   }
 
+  // Try ConstantPointerNull
+  if (LLVMIsAConstantPointerNull(Cst)) {
+    check_value_kind(Cst, LLVMConstantPointerNullValueKind);
+    LLVMTypeRef Ty = TypeCloner(M).Clone(Cst);
+    return LLVMConstNull(Ty);
+  }
+
   // Try undef
   if (LLVMIsUndef(Cst)) {
     check_value_kind(Cst, LLVMUndefValueValueKind);
     return LLVMGetUndef(TypeCloner(M).Clone(Cst));
   }
 
   // Try null
   if (LLVMIsNull(Cst)) {