[clang] [Sema] Skip ExternalSource query and clear DeleteExprs in incremental mode (PR #192856)

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lucasvallejoo wrote:

Hi @vgvassilev, you are right. My apologies for the messy history—I had some leftover commits from my previous work.

I have just performed a force-push to squash everything into a single, independent commit containing only the Sema changes and the regression test. It should be ready for review now. Thanks!

https://github.com/llvm/llvm-project/pull/192856