Auto merge of rust-lang#2985 - RalfJung:retag-fields, r=saethlin

make full field retagging the default

The 'scalar' field retagging mode is clearly a hack -- it mirrors details of the codegen backend and how various structs are represented in LLVM. This means whether code has UB or not depends on surprising aspects, such as whether a struct has 2 or 3 (non-zero-sized) fields. Now that both hashbrown and scopeguard have released fixes to be compatible with field retagging, I think it is time to enable full field retagging by default.

`@saethlin` do you have an idea of how much fallout enabling full field retagging by default will cause? Do you have objections to enabling it by default?

Fixes rust-lang/miri#2528

Author: bors