FIXME: Drop glue takes unsized values differently than other functions

[nikomatsakis]

Currently drop glue takes sized values as a *(data, meta), but elsewhere in MIR we pass (data, meta) as two separate arguments. This is hokey and introduces a lot of pain to convert back and forth. I've tried to tag many of those areas with this issue number.

cc @eddyb

[Mark-Simulacrum]

I have a branch that in theory fixes this, but it's based on #38302. Will post PR once that gets merged.