Auto merge of rust-lang#12837 - DorianListens:dscheidt/generate-enum-data, r=Veykril

feat: support associated values in "Generate Enum Variant" assist

This change adds support for associated values to the "Generate Enum Variant" assist.

I've split the implementation out into 4 steps to make code review easier:
- Add "add_variant" support to the structural ast editing system in `edit_in_place`
- Migrate `generate_enum_variant` to use structural ast editing instead of string manipulation
- Support tuple fields
- Support record fields

Please let me know if I should leave the commits as-is, or squash before merging.

Fixes rust-lang#12797

Author: bors