[OptTable] Make ValuesCode initialisation of Options constexpr

[OptTable] Make ValuesCode initialisation of Options constexpr

Current implementation requires a copy of the initialization array to a
vector to be able to modify their Values field.

This is inefficient: it requires a large copy to update a value, while
TableGen has all information to avoid this overwrite.

Modify TableGen to emit the Values code and use it to perform the
initialisation.

The impact on performance is not amazing compared to the actual
compilation, but still noticeable:

https://llvm-compile-time-tracker.com/compare.php?from=d9ab3e82f30d646deff054230b0c742704a1cf26&to=f2b37fb65d5149f70b43d1801beb5239285a2a20&stat=instructions:u

Differential Revision: https://reviews.llvm.org/D140699