[ADT] Drop llvm::Optional clang-specific optmization for trivially copyable types

[tstellar]

Summary

This fixes libLLVM.so ABI mismatches between llvm compiled with clang
and llvm compiled with gcc (PR39427).

[bkramer]

Thanks. There's a bit of unused code that passes isPodLike through the layers, but this is the minimal fix. Looks good to me.

[rjmccall]

Hmm. This makes Optional<int> non-trivial, which is a serious semantic problem for certain uses of the type (e.g. putting it in a union); it's not just an optimization. Obviously those were unportable because of the #if, but we should really fix this. There's no way that GCC just generically miscompiles all partial specializations.

[rjmccall]

llvm/trunk/include/llvm/ADT/Optional.h

121

I wonder if GCC's problem is that we haven't formally created an object of type T at this address, and the assignment isn't good enough We could just placement-new here; you're allowed to just replace objects like that when the destructor is trivial.

llvm/trunk/include/llvm/ADT/Optional.h

T *getPointer() {
     return reinterpret_cast<T *>(storage.buffer);
   }
   const T *getPointer() const {
     assert(hasVal);
     return reinterpret_cast<const T *>(storage.buffer);
   }
 };
 
-#if !defined(__GNUC__) || defined(__clang__) // GCC up to GCC7 miscompiles this.
-/// Storage for trivially copyable types only.
-template <typename T> struct OptionalStorage<T, true> {
-  AlignedCharArrayUnion<T> storage;
-  bool hasVal = false;
-
-  OptionalStorage() = default;
-
-  OptionalStorage(const T &y) : hasVal(true) { new (storage.buffer) T(y); }
-  OptionalStorage &operator=(const T &y) {
-    *reinterpret_cast<T *>(storage.buffer) = y;
-    hasVal = true;
-    return *this;
-  }
-
-  void reset() { hasVal = false; }
-};
-#endif
 } // namespace optional_detail
 
 template <typename T> class Optional {
   optional_detail::OptionalStorage<T, isPodLike<T>::value> Storage;
 
 public:
   using value_type = T;