[libc][NFC] add arrow operator to optional

[sivachandra]

Good to go after adding smoke tests.

libc/src/__support/CPP/optional.h

78

For consistency with operator*, make it const & and add a non-const & flavor also.

[michaelrj]

add tests and non-const version

libc/src/__support/CPP/optional.h

78

If I change this to be a reference, then to use the -> operator the contained object must also support the -> operator. Here's the error message when I try it:
error: member reference type 'const __llvm_libc::cpp::StringView' is not a pointer

I think it's more useful to leave it as is, so that it works as expected without additional effort.

libc/src/__support/CPP/optional.h

public:
   constexpr optional &operator=(nullopt_t) {
     reset();
     return *this;
   }
 
   constexpr T &operator*() & { return Storage.StoredValue; }
 
   constexpr const T &operator*() const & { return Storage.StoredValue; }
+
+  constexpr const T *operator->() const { return &Storage.StoredValue; }
 };
 } // namespace cpp
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_SUPPORT_CPP_OPTIONAL_H