fix: broken C emitted when re-assigning a unit-typed variable

[iwami4438]

• set! always emitted an assignment, producing invalid C (x = ;) for unit-typed (()) variables, which have no C declaration
• Now skipped as a no-op, matching declaration behavior (Emit.hs 624~)
• Added a regression test
• Ref: Reassigning unit values emits broken C #1542

[hellerve]

looks good, thanks! in the future it might be better to separate the reformat from the fix. i had to dig for a while to understand the magic is in src/Emit.hs:625.

[iwami4438]

Thank you for the review, and sorry for the extra digging it caused.
From now on, I'll use git commit --no-verify to keep the reformat separate from the logic change.

[hellerve]

i wonder why my commit hooks don’t seem to fire the same way. we should revisit the ormolu setup in general i guess! i’ll open an issue.

[iwami4438]

I'm using the version I installed this way. stack install ormolu
However, it does feel a little intrusive.
For reference, here's the version I'm using:

ormolu -v
ormolu 0.7.2.0
using ghc-lib-parser 9.6.7.20250325

stack --version
Version 3.11.1, Git revision 2352d78a8ac5b42d021c8064b8f64ac1c8b8b3d5 x86_64 hpack-0.39.6