[basic.link], [expr.ref] Harmonize font for tuple representing direct base class relationship

[eisenwave]

Fixes NB US 1-405 (C++26 CD).

Fixes cplusplus/nbballot#586.

This only changes the spellings in core wording, in [basic.link] and [expr.ref].

@jensmaurer requested in cplusplus/nbballot#586 (comment) that code font should not be used. That seems to be the path of least resistance, although, to be fair, a direct base class relationship is always between two types, and we tend to use code font for types. Using math font also leads to a quirky change in [expr.ref] where \tcode{T1} becomes $\mathit{T1}$.

In [meta.reflection], the wording uses $(D, B)$ consistently everywhere. This spelling also appears once in https://eel.is/c++draft/expr.ref#8.6. I think we should aim to spell both the base class relationship and the data member description tuples entirely in math context, including the parentheses, which is already the most common form.

Maybe we could revisit these font choices later, but for now, let's just make it consistent with a fairly small change, and move on.

[jensmaurer]

I'd suggest to leave the code font for T1 alone and say in the last line:

"or a direct base class relationship $(D, B)$ where $D$ is the type \tcode{T1}."

That seems least awkward from a "types are code font" perspective.

@tkoeppe , what do you think?