data_type_for() is a function template that converts a C++
type to a database dynamic type (data_type object).
Instead of implementing a function per type, implement a variable
template instance. This is shorter and nicer.
Since the original type variables (e.g. long_type) are defined separately,
use a reference instead of copying to avoid initialization order problems.
To catch misuses of data_type_for the general data_type_for_v variable
template maps to some unused tag type which will cause a build error
when instantiated.
The original motivation for this was to allow for partial
specialization of data_type_for() for tuple types, but this isn't
really workable since the native type for tuples is std::vector<data_value>,
not std::tuple, and I only checked this after getting the work done,
so this isn't helping anything; it's just a little nicer.
Closes#13043
3042deb930