Rollup merge of #124797 - beetrees:primitive-float, r=davidtwco

Refactor float `Primitive`s to a separate `Float` type

Now there are 4 of them, it makes sense to refactor `F16`, `F32`, `F64` and `F128` out of `Primitive` and into a separate `Float` type (like integers already are). This allows patterns like `F16 | F32 | F64 | F128` to be simplified into `Float(_)`, and is consistent with `ty::FloatTy`.

As a side effect, this PR also makes the `Ty::primitive_size` method work with `f16` and `f128`.

Tracking issue: #116909

`@rustbot` label +F-f16_and_f128

Author: matthiaskrgr