SwiftPM and Swift Build tests hang on Windows when using 6/12 nightly main snapshot

[owenv]

Description

SwiftPM and Swift Build tests hang on Windows when using 6/12 nightly main snapshot. Root cause is unclear at the moment, I haven't been able to get much useful info out of a process dump.

Reproduction

swift test in the swift-build repo is the simplest reproducer

Expected behavior

Testing completes

Environment

6/12 nightly main snapshot on Windows

Additional information

[owenv]

The 6/9 snapshot allows tests to complete. Trying some speculative reverts in #79684