Pull request 994 in include-what-you-use: [cmake] Include headers correctly for in-tree builds.

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New pull request 994 by bc-lee: [cmake] Include headers correctly for in-tree builds.
https://github.com/include-what-you-use/include-what-you-use/pull/994

When trying to build in-tree, I found that neither
LLVM_INCLUDE_DIRS nor CLANG_INCLUDE_DIRS were not defined.
Fix it by manually including the correct headers.

Also, add a guide for building IWYU with LLVM source code.

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Comment #0 on pull request 994 by kimgr: [cmake] Include headers correctly for in-tree builds.
https://github.com/include-what-you-use/include-what-you-use/pull/994

Thanks for this, I didn't know about LLVM external projects!

Hmm, I wonder if we should retire the IWYU in-tree build entirely. It hasn't been officially supported for a few years, and the external project hook seems nicer to integrate with an LLVM tree.

I need to think a little bit about how to best expose this to users.

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Comment #1 on pull request 994 by bc-lee: [cmake] Include headers correctly for in-tree builds.
https://github.com/include-what-you-use/include-what-you-use/pull/994

Well, at least I'm one of the users of in-tree builds, so I hope IWYU supports in-tree builds. 😁 Also, not everyone in the world is using the pre-built LLVM.
There are many projects that support in-tree builds. https://cs.github.com/?scopeName=All+repos&scope=&q=LLVM_EXTERNAL_PROJECTS+%28lang%3ACMake+OR+lang%3AMarkDown%29
If you don't mind, I'd like to contribute to a Github action script to test in-tree builds.