change how we handle unions of missings and other types in unitary functions

[oxinabox]

Follow up to #53602 (comment)
all the related need_to_handle_undef_sparam should now pass if this is right

[oscardssmith]

is this ready to merge?

[oxinabox]

It seems the CI failures are legit

[martinholters]

Aren't the ::Type{Missing} methods still needed? (Or a branch to deal with T==Missing to avoid calling nonmissingtype_checked on it.)
OTOH, the ::Type{Any} might not be needed anymore to prevent a stack overflow, as the nonmissingtype_checked will throw in the ::Type{>:Missing} method. (But the more specific error might still be worth it.)