Rollup merge of #67122 - petrochenkov:nodedup, r=estebank

Rollup merge of #67122 - petrochenkov:nodedup, r=estebank

Do not deduplicate diagnostics in UI tests

Error reporting infrastructure deduplicates identical diagnostics with identical spans.

While it's preferable to do this in "release"/"user-facing" mode, it sometimes brings [confusion](https://github.com/rust-lang/rust/pull/50682#issuecomment-390949878) and hides details that may be important during development.

Do we run some passes multiple times when we could do it once?
How many times we run them exactly? Can this number be large? Can the multiplied error construction be expensive? Can speculative checks be made cheaper if they don't report errors?

*Relying* on this mechanism to deduplicate some specific error never looks like a proper solution to me personally.

In this PR I attempt to disable this deduplication by applying `-Z deduplicate-diagnostics=no` to UI tests.