[PATCH v1] Merge common attribute handling code between libgccjit and c-family.

[Guillaume Gomez via Sourceware Forge]

I'll take that as you globally approving the patch. Considering that it moves a lot of code around, it's very prone to bitrot. I'm fixing the merge conflict, but if another merge conflict happens before next review, I'll just wait for you to review the broken state as it's time consuming to keep up.

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https://forge.sourceware.org/gcc/gcc/pulls/207#issuecomment-7382