Convert brand-pointer to constexpr-function (1/3)
Convert brand-pointer to constexpr-function (1/3)

MSVC erroneously refuses to compile code which calculates the address of
objects of static storage duration at compile-time, despite this being
legal. Work around this by changing `_capnpPrivate::brand` from constexpr
pointers to static constexpr functions returning pointers. The compiler
is then free to calculate it at compile-time if it can (gcc, clang), but
can also defer it to run-time if it can't (MSVC).

This change is split into three commits:

1. Changes to the code generator (this commit).
2. A bootstrap regeneration, after which the codebase will not compile.
3. Changes to the generated code support header, after which the codebase
will again compile.

mologie/capnproto,mologie/capnproto,mologie/capnproto