[TableGen] Add missing std::moves to fix build failure.

gcc 4.7 seems to disagree with gcc 5.3 about whether you need to say
'return std::move(thing)' instead of just 'return thing'. All the
json::Arrays and json::Objects that I was implicitly turning into
json::Values by returning them from functions now have explicit
std::move wrappers, so hopefully 4.7 will be happy now.

llvm-svn: 336772