[flang] fix: split ifndef for CHECK and CHECK_MSG

[ivan-aksamentov]

Resolves #114703

I think it's the best practice that each macro has it's own ifndef check and this way the build issue is resolved for me.

I also find the names of these macro a bit too generic - an easy recipe for conflicts. In my case, the error was likely caused by something else defining CHECK but not CHECK_MSG, so likely these CHECK and CHECK_MSG weren't actually working at all because the result of ifndef is always false.

As a definitive fix, perhaps it makes sense to rename them to something more specific, e.g. FLANG_CHECK and FLANG_CHECK_MSG.

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[mgorny]

This change makes sense to me, and fixes the build failure that I'm hitting with GCC.

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