Add bounds check when allocating TweakDB data

[psiberx]

• Added bounds check for flat value allocations. The hard limit for TweakDB data buffer is 16 MiB.
• Fixed static assertions in Variant for C++20 and older. Before C++23 static_assert(false, ...) will always cause compilation error, even in discarded branches. For C++20 and older, the condition in discarded branch must depend on any template parameter.

[wopss]

Thanks!