WithHelperKey breaks on clang with asserts on (#26500)

`WithHelperKey` breaks on clang with asserts

`WithHelperKey` is a template class that has an inline field that it
uses to produce a memory address that is used as a data key. This PR is
not concerned with the underlying causes for this unconventional design.
Rather, this change aims to fix a clang crash that happens whenever
nearly daily when building brave with a clang binary that has asserts on.
In particular, it looks like some combination of the key address and the
use `std::addressof` cause clang to assert and crash.

This change works around this crash by returning the address of the
inline variable using `&`. For a repro of the crash, see:

  https://godbolt.org/z/M6oq6b3sz

An issue has been created for this issue as well

  llvm/llvm-project#111188

components/brave_rewards/core/rewards_engine_helper.h

@@ -47,7 +47,7 @@ class RewardsEngineHelper : public base::SupportsUserData::Data {
 template <typename T>
 class WithHelperKey {
  public:
-  static const void* GetHelperKey() { return std::addressof(kHelperKey); }
+  static const void* GetHelperKey() { return &kHelperKey; }
 
  private:
   static inline const int kHelperKey = 0;