Adjust r316292 - remove the anonymous union for sharing a bitfield in FunctionDecl.

The anonymous union did NOT save us storage, but instead behaved as if we added an additional integer data member to FunctionDecl.  

For additional context, the anonymous union renders the bit fields as non-adjacent and prevents them from sharing the same 'memory location' (i.e. bit-storage) by requiring the anonymous union object to be appropriately aligned.

This was confirmed through discussion with Richard Smith in Albuquerque (ISO C++ Meeting)

https://reviews.llvm.org/rL316292

llvm-svn: 317984