Add mir_borrowck_with_facts to retrieve borrow-checking results and Polonius facts in a single pass

[stellanomia]

This PR introduces a new API mir_borrowck_with_facts to rustc_borrowck::consumers. This allows compiler consumers to extract both the standard mir_borrowck query results and Polonius facts in a single borrow-checking pass.

Currently, external consumers (like custom drivers or analyzers) have no efficient way to retrieve both the standard mir_borrowck result and Polonius facts. Using get_bodies_with_borrowck_facts discards the borrow check result, and the standard query is not capable of retrieving Polonius facts.

Furthermore, consumers cannot reimplement mir_borrowck locally to capture facts because BorrowCheckRootCtxt is private. While this encapsulation of unstable borrow-checker internals makes sense, it currently forces a redundant second pass when a tool needs both facts and query results to continue the compilation.

This PR adds mir_borrowck_with_facts to let consumers get both in a single pass. This is particularly useful for tools that override the mir_borrowck query to perform analysis but still need to continue the compilation.

I also updated tests/ui-fulldeps/obtain-borrowck.rs to use the new API. This avoids the need to manually instantiate a default provider just to call the original mir_borrowck a second time.

Changes:

• Add BorrowckResultWithFacts to bundle results.
• Add mir_borrowck_with_facts to the consumers module.
• Update obtain-borrowck.rs test to use the new API.

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[davidtwco]

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