stage2: more in-memory coercion

[Snektron]

• In-memory coercion for function types.
• Moved coercion of error sets from the switch in Sema.coerce to sema.coerceInMemoryAllowed. This should be OK considering all error set types are currently u16.
• In-memory coercion for error unions.

I also moved a few tests over to stage 2 which worked, though i don't think all of them were related to these changes.

[Snektron]

I don't think this this ci failure is related to my changes

[andrewrk]

I believe the CI failure is #9439

In general, I'm happy to merge stage2 PRs prior to CI checks completing if you have done the following locally:

./zig-out/bin/zig test -fLLVM ../test/behavior.zig -I ../test
./zig-out/bin/zig test -ofmt=c ../test/behavior.zig -I ../test
./zig build test-stage2 -Denable-qemu -Denable-llvm -Denable-wasmtime -Dskip-compile-errors
./zig build -Dtarget=arm-linux-musleabihf # catch int casting compile errors for 32-bit builds

Should only take a couple minutes to run all these checks.

Changes look good 👍 merging now.