[TableGen] Fix std::array initializer to avoid warnings with older tool chains. NFC
A std::array is implemented as a template with an array inside a struct.
Older versions of clang, like 3.6, require an extra set of curly braces
around std::array initializations to avoid warnings.
The C++ language was changed regarding this by CWG 1270. So more modern
tool chains does not complain even if leaving out one level of braces.