fix Expected<..., eUnion> move / copy ctor

Summary:
Consider `Expected<..., eUnion>(Expected&&)` ctor, where
`ExpectedUnion` move ctor would be called first, which is noop leaving
`which_`  uninitialized; then `MoveConstructible` ctor is executed, calling
`assignValue()`, which performs action depending on uninitialized `which_`.

Reviewed By: yfeldblum

Differential Revision: D4073199

fbshipit-source-id: 623660e7047afcebf9f72d83f91f84ff6078090f

folly/Expected.h

@@ -249,12 +249,11 @@ struct ExpectedUnion {
   union {
     Value value_;
     Error error_;
-    char ch_;
+    char ch_{};
   };
-  Which which_;
+  Which which_ = Which::eEmpty;
 
-  explicit constexpr ExpectedUnion(EmptyTag) noexcept
-      : ch_{}, which_(Which::eEmpty) {}
+  explicit constexpr ExpectedUnion(EmptyTag) noexcept {}
   template <class... Vs>
   explicit constexpr ExpectedUnion(ValueTag, Vs&&... vs) noexcept(
       noexcept(Value(static_cast<Vs&&>(vs)...)))