user_context has inconsistent const qualifiers

[steven-johnson]

Function declarations (from CodeGen_C.cpp) emit function prototypes with it declared as "const void_", but all the runtime callbacks declare it as "void_".

[steven-johnson]

Update, several years later: amazingly, this usually isn't an issue, but:

• If a define_extern() call references Halide-generated AOT code directly, and
• The define_extern() call passes a user_context, and
• c_plus_plus_name_mangling is enabled (for both caller and callee),

...we get link errors because of this dichotomy, since the caller expects void * but callee is defined as void const *.

Fixing this "correctly" (ie so that everything always uses identical types for user_context in all cases) will require a lot of downstream churn. Since the existing user_context design is already considered to be suboptimal in a number of ways (see #7298), it doesn't really make sense to try to spend effort to update this without addressing the other issues. #7291 is a somewhat awkward workaround to "fix" the link errors mentioned above, but definitely no better than a workaround.