Rollup merge of #33339 - oli-obk:fix/const_eval, r=japaric

fix various const eval errors

These were found after const_evaluating arbitrary expressions and linting if the const evaluator failed

fixes #33275 (int -> float casts for negative ints)
fixes #33291 (int -> char casts (new! wasn't allowed in constants until this PR))

r? @eddyb

cc @bluss @japaric

diff --cc src/librustc_const_eval/eval.rs
Simple merge