…evert swiftlang#931)

Revert "Merge pull request swiftlang#950 from stmontgomery/revert-issue-severity" (swiftlang#950)

This reverts commit 9998633, reversing
changes made to 55d0023.

…evert #931) (#952)

This un-reverts #950, effectively reintroducing the changes recently
landed in #931.

The revert was needed because it revealed a latent bug in the Swift
compiler, tracked by swiftlang/swift#79304. I
reproduced that failure and included a workaround in the second commit
on this PR.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

…#951)

This modifies `Package.swift` to enable Library Evolution for builds of
the package.

### Motivation:

I recently landed a change (#931) which passed our project-level CI but
later failed in Swift CI. The difference ended up being due to the
latter building with Library Evolution (LE) enabled, whereas our
project-level CI builds via SwiftPM and does not enable LE. The change
was reverted (#950) but this revealed a gap in our testing strategy. We
should always build these targets with LE enabled.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Fixes rdar://144655439