GH-35526: [CI][C++] Fixing arrow::internal::IsNullRunEndEncoded redec…

…lared (#35527)

### Rationale for this change

Fixing arrow::internal::IsNullRunEndEncoded redeclared. The CI will report:

```
'bool arrow::internal::IsNullRunEndEncoded(const arrow::ArrayData&, int64_t)' redeclared without dllimport attribute after being referenced with dll linkage
```

And:

```
warning: 'selection_vector_type' may be used uninitialized
```

### What changes are included in this PR?

1. Add `ARROW_FRIEND_EXPORT` for friend function
2. Initialize pointer in gandiva with `nullptr`

### Are these changes tested?

No

### Are there any user-facing changes?

No

* Closes: #35526

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/src/arrow/array/data.h

@@ -527,7 +527,8 @@ struct ARROW_EXPORT ArraySpan {
   int64_t ComputeLogicalNullCount() const;
 
  private:
-  friend bool internal::IsNullRunEndEncoded(const ArrayData& span, int64_t i);
+  ARROW_FRIEND_EXPORT friend bool internal::IsNullRunEndEncoded(const ArrayData& span,
+                                                                int64_t i);
 
   bool IsNullSparseUnion(int64_t i) const;
   bool IsNullDenseUnion(int64_t i) const;

cpp/src/gandiva/llvm_generator.cc

@@ -279,7 +279,7 @@ Status LLVMGenerator::CodeGenExprValue(DexPtr value_expr, int buffer_count,
   arguments.push_back(types()->i64_ptr_type());  // offsets
   arguments.push_back(types()->i64_ptr_type());  // bitmaps
   arguments.push_back(types()->i64_ptr_type());  // holders
-  llvm::Type* selection_vector_type;
+  llvm::Type* selection_vector_type = nullptr;
   switch (selection_vector_mode) {
     case SelectionVector::MODE_NONE:
     case SelectionVector::MODE_UINT16: