[clang-tools-extra] [clang-doc] create a separate cmake file for clang-doc's lit tests (PR #165935)

[Aiden Grossman via cfe-commits]

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+# Specialize the clang-doc target to avoid building other projects
+add_lit_testsuite(check-clang-extra-clang-doc "Running clang-doc tests"
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boomanaiden154 wrote:

It's not actually excluded from the all target. That specific test suite needs to be excluded from `check-all` because otherwise it will run twice. Whether any buildbots run that target specifically I'm not sure. I wouldn't bet on it, but definitely is possible.

https://github.com/llvm/llvm-project/pull/165935