[flang] [llvm] [clang] [flang][OpenMP] Add semantic check for declare target (PR #72770)

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shraiysh wrote:

> Might be worth asking someone from the Clang team to have a look at the Clang changes if you're a little unsure on them, but if you're happy with them can disregard doing that!

Thank you for the review @agozillon @skatrak. I am not sure who I should request the review from, but I have requested a review from the codeowner of OpenMP conformance. clang:openmp is also tagged as a topic in this patch, so I will wait for 1-2 days before merging this if someone from clang/openmp team has any concerns with this. 

https://github.com/llvm/llvm-project/pull/72770