gh-94673: Fix compiler warning in typeobject.c

[erlend-aasland]

static_builtin_index_is_set() is only used in asserts; inline it to avoid compiler warnings.

• Issue: [subinterpreters] Static types incorrectly share some objects between interpreters #94673

[ericsnowcurrently]

Thanks for working on this. I have one suggestion for handling it a different way, which preserves the intention of the original code.

[ericsnowcurrently]

Thanks!