[Support][JSON][NFC] Silence GCC warning about broken strict aliasing rules

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.

Using two static_cast via void * instead of reinterpret_cast
silences the warning and presumably makes GCC understand that no
strict-aliasing violation is happening.

No functional change intended.

Patch by: kimgr (Kim Gräsman)

Reviewers: sammccall, xiangzhai, HaoLiu, llvm-commits, xbolva00

Reviewed By: sammccall, xbolva00

Subscribers: xbolva00

Differential Revision: https://reviews.llvm.org/D50608

llvm-svn: 339521