NFC: bit.h don't warn on strict aliasing for GCC <= 7.1

[jfb]

Summary

Addressed https://bugs.llvm.org/show_bug.cgi?id=38885

[jfb]

I tried this out with a few GCC and clang versions to make sure the pragma was supported by both. Seems fine.

[jfb]

Assuming this is good to go since it's a build breakage. Happy to do follow-ups if you have post-commit comments.

[xbolva00]

Please check also https://reviews.llvm.org/rL339521, maybe we can apply that workaround for GCC here too :)

llvm/trunk/include/llvm/ADT/bit.h

 
 template <typename To, typename From,
           typename = typename std::enable_if<sizeof(To) == sizeof(From)>::type,
           typename = typename std::enable_if<isPodLike<To>::value>::type,
           typename = typename std::enable_if<isPodLike<From>::value>::type>
 inline To bit_cast(const From &from) noexcept {
   alignas(To) unsigned char storage[sizeof(To)];
   std::memcpy(&storage, &from, sizeof(To));
+#if defined(__GNUC__)
+  // Before GCC 7.2, GCC thought that this violated strict aliasing.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-aliasing"
+#endif
   return reinterpret_cast<To &>(storage);
+#if defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
 }
 
 } // namespace llvm
 
 #endif