Rollup merge of #124110 - beetrees:neg-f16-f128, r=compiler-errors

Fix negating `f16` and `f128` constants

Make `f16` and `f128` constants respect `neg` in `parse_float_into_scalar`.

Tracking issue: #116909

```@rustbot``` label +F-f16_and_f128

Author: workingjubilee