Rollup merge of #67709 - petrochenkov:nodedup2, r=Centril

Rollup merge of #67709 - petrochenkov:nodedup2, r=Centril

Introduce an option for disabling deduplication of diagnostics

With the intent of using it in UI tests (https://github.com/rust-lang/rust/pull/67122).

The option is boolean (`-Z deduplicate-diagnostics=yes/no`) and can be specified multiple times with later values overriding earlier values (`-Z deduplicate-diagnostics=no -Z deduplicate-diagnostics=yes` == `-Z deduplicate-diagnostics=yes`), so it can be set in a hierarchical way, e.g. UI testing infra may disable the deduplication by default with specific tests being able to enable it back.