[clang] [Clang] fix crash by avoiding invalidation of extern main declaration during strictness checks (PR #104594)

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https://github.com/Sirraide approved this pull request.

Ah, I should have noticed that `setInvalidDecl()` makes no sense there when I reviewed that. LGTM after fixing what Corentin pointed out.

https://github.com/llvm/llvm-project/pull/104594