warning(module): unexpected EOF reading cached ZIR when running test-cases multi-threaded

[andrewrk]

Now that zig build test-cases runs the cases multi-threaded, it exposed a bug. This manifested as a CI failure on 27610b0 which is unrelated to the actual changes.

====== expected this output: =========
␃

======== instead found this: =========
warning(module): unexpected EOF reading cached ZIR for Thread/Condition.zig
␃

======================================
====== expected this output: =========
␃

======== instead found this: =========
warning(module): unexpected EOF reading cached ZIR for math/modf.zig
warning(module): unexpected EOF reading cached ZIR for math/isnormal.zig
␃

======================================
safety/switch on corrupted union value (stage1, native) failed: TestExpectedEqual
safety/unsigned-signed vector cast (stage1, native) failed: TestExpectedEqual
2 tests failed

Let's inspect the pipeline with regards to AstGen and caching and look for any possibility of this error occurring.

[motiejus]

I noticed similar "unexpected EOF" (for different files) in #13160 and suspect this may ho away with #13342.

[jacobly0]

This sure looks a heck of a lot like the bug fixed by #14821.