* std.meta: correct use of `default_value` in reification. stage1
   accepted a wrong type for `null`.
 * Sema: after instantiating a generic function, if the return type ends
   up being a comptime-known type, then we return an error, undoing the
   generic function instantiation, and making a comptime function call
   instead.
   - We also needed to clean up the dependency graph in this case.
 * Sema: reified enums set tag_ty_inferred to false since an integer tag
   type is provided. This is a limitation of the `@Type` builtin which
   will be addressed with #10710.
 * Sema: fix resolveInferredErrorSet incorrectly calling
   ensureFuncBodyAnalyzed on generic functions.