C front-end: support GCC attributes before contract clauses

[tautschnig]

Add a rule to also permit GCC attributes (C23 attributes already worked) before contract clauses attached to functions. This is at the expense of creating additional shift/reduce conflicts in our parser.

Fixes: #8685

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[kroening]

Add a rule to also permit GCC attributes (C23 attributes already worked) before contract clauses attached to functions. This is at the expense of creating additional shift/reduce conflicts in our parser.

Please don't -- at great pain I have so far managed to keep conflicts out of the grammar!

One you start accumulating these, they become impossible to debug.