fix some crashy behavior of invoke with builtins

[adienes]

builtins have empty method tables so invoke will segfault or otherwise crash (see some tests). this PR attempts to fix that

[adienes]

I guess this breaks trimming. would it be better for this to just be always an error can't invoke builtins or something, rather than trying to make it work?

[adienes]

this now solely turns internal errors into surface errors so I'm removing the feature and news label (although docs probably good here)

[adienes]

with the clarification in Functions.md that builtins are not generic, I think the docstring of invoke no longer needs updating (as it specifics generic functions). but I can tag triage to be sure, unless this gets definitively decided before the next meeting

[vtjnash]

They are generic functions (or rather just functions, since Julia doesn't have any non-generic function). There's a mistake that slipped in to jl_specializations_get_linfo_ though, since it is supposed to return unspecialized if source and generated are both NULL, but it instead checks if it is a non-generic function (sig == jl_anytuple_type), but we don't have any non-generic functions.

[adienes]

thanks for the advice. does the latest version look closer to what you had in mind?