Silence GCC warning about broken strict aliasing rules

[kimgr]

Summary

The as<T>() method would trigger the following warning on GCC <7:

warning: dereferencing type-punned pointer will break
    strict-aliasing rules [-Wstrict-aliasing]

  return *reinterpret_cast<T *>(Union.buffer);
                                            ^

Union.buffer is guaranteed to be aligned to whatever types it contains,
and json::Value maintains the invariant that it only calls as<T>() for a
T it has previously placement-newed into Union.buffer. This should
follow the rules for strict aliasing.

Older GCCs might be incorrectly identifying this as a strict-aliasing
violation and undefined behavior, which could lead to misoptimizations
down the line. We've seen no evidence of such behavior.

No functional change intended.

[kimgr]

Better solution in D50608.

include/llvm/Support/JSON.h

private:
   void moveFrom(const Value &&M);
   friend class Array;
   friend class Object;
 
   template <typename T, typename... U> void create(U &&... V) {
     new (reinterpret_cast<T *>(Union.buffer)) T(std::forward<U>(V)...);
   }
   template <typename T> T &as() const {
+#if defined(__GNUC__) && !defined(__clang) && (__GNUC__ < 7)
+// Silence
+//
+//   warning: dereferencing type-punned pointer will break strict-aliasing rules
+//   [-Wstrict-aliasing]
+//
+// This private method is always called after type-checking, so the
+// reinterpret_cast<> just re-casts the char buffer to the type that was
+// placement-new-ed into it. Because AlignedCharArrayUnion guarantees correct
+// alignment for all contained types, this plays by the strict aliasing rules.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-aliasing"
+#endif
     return *reinterpret_cast<T *>(Union.buffer);
+#if defined(__GNUC__) && !defined(__clang) && (__GNUC__ < 7)
+#pragma GCC diagnostic pop
+#endif
   }
 
   template <typename Indenter>
   void print(llvm::raw_ostream &, const Indenter &) const;
   friend struct llvm::format_provider<llvm::json::Value>;
 
   enum ValueType : char {
     T_Null,