Make #[cfg(version)] respect RUSTC_OVERRIDE_VERSION_STRING
The `#[cfg(version(...))]` feature is currently under-tested. Part of it is the difficulty that it is hard to write a test that never changes, while the version of the Rust compiler indeed *does* change.
PR #81468 added the first and so far only test of `#[cfg(version(...))]`'s functionality (there is one other test for the *syntax*, that also acts as feature gate). But that test uses a proc macro that parses the version: the text of the test doesn't contain the actual `#[cfg(version(...))]`.
This PR makes `#[cfg(version(...))]` respect `RUSTC_OVERRIDE_VERSION_STRING`, added by PR #124339, allowing us to virtually pin the rustc version and write tests from all directions against some specific version.
The PR also adds a functional test of `#[cfg(version(...))]` that leverages `RUSTC_OVERRIDE_VERSION_STRING`.
Pulled out of #141137.
Tracking issue: #64796
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