[ADT] Deprecate Optional::getPointer

[kazu]

Summary

This patch deprecates Optional::getPointer. Note that I've converted
all known occurrences of X.getPointer() to &*X on Nov 21, 2022 in
commit 1f914944b6c9a5e4229ceb9f06140fdf178c5ea0.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

llvm/include/llvm/ADT/Optional.h

public:
   Optional &operator=(const T &y) {
     Storage = y;
     return *this;
   }
   Optional &operator=(const Optional &O) = default;
 
   void reset() { Storage.reset(); }
 
+  LLVM_DEPRECATED("Use &*X instead.", "&*X")
   constexpr const T *getPointer() const { return &Storage.value(); }
+  LLVM_DEPRECATED("Use &*X instead.", "&*X")
   T *getPointer() { return &Storage.value(); }
   constexpr const T &value() const & { return Storage.value(); }
   T &value() & { return Storage.value(); }
 
   constexpr explicit operator bool() const { return has_value(); }
   constexpr bool has_value() const { return Storage.has_value(); }
-  constexpr const T *operator->() const { return getPointer(); }
-  T *operator->() { return getPointer(); }
+  constexpr const T *operator->() const { return &Storage.value(); }
+  T *operator->() { return &Storage.value(); }
   constexpr const T &operator*() const & { return value(); }
   T &operator*() & { return value(); }
 
   template <typename U> constexpr T value_or(U &&alt) const & {
     return has_value() ? value() : std::forward<U>(alt);
   }
 
   /// Apply a function to the value if present; otherwise return None.