cmake: raise Clang's expression nesting limit

[jcelerier]

Clang caps expression nesting at 256. Avendish assembles the module list with std::tuple_cat, and libstdc++'s tuple constructor folds over every element:

explicit(!(__is_implicitly_default_constructible_v<_Elements> && ...))

giving one term per module. Once the count passes 256 that stops being a diagnostic and becomes a hard error:

/usr/include/c++/14/tuple:965:72: fatal error: instantiating fold expression with 330 arguments exceeded expression nesting limit
  965 |       explicit(!(__is_implicitly_default_constructible_v<_Elements> && ...))
note: in instantiation of template class 'std::tuple<halp::midi_bus<...>, ...>'
note: in instantiation of function template specialization 'std::tuple_cat<...>'
      3rdparty/avendish/include/avnd/concepts/modules.hpp:217

It is at 330 today and only grows as plug-ins are added, so this sets the ceiling well clear rather than just above the current count.

-fbracket-depth is Clang-only and costs nothing when the limit is not reached; GCC has no equivalent cap, so GCC builds are unaffected.

Seen on the clang-20 Ubuntu plucky legs of #2176, #2179, #2181, #2182 and #2183 — 8 failing jobs across those PRs.