`InferCtxt::tainted_by_errors_flag` should be `Option<ErrorGuaranteed>`

[lcnr]

rust/compiler/rustc_infer/src/infer/mod.rs

Line 319 in 5af97e8

tainted_by_errors_flag: Cell<bool>,

should be Cell<Option<ErrorGuaranteed>> to guarantee that we emit an error when that flag is set. This is already done for tainted_by_errors of other types in rustc.

rust/compiler/rustc_middle/src/ty/context.rs

Line 537 in 5af97e8

pub tainted_by_errors: Option<ErrorGuaranteed>,

[chenyukang]

@rustbot claim

[chenyukang]

tainted_by_errors_flag renamed to tainted_by_errors to keep consistent.