[llvm] [clang] [CMake][PGO] Add option for using an external project to generate profile data (PR #78879)

[Petr Hosek via cfe-commits]

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@@ -15,7 +19,7 @@ if(LLVM_BUILD_INSTRUMENTED)
     )
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petrhosek wrote:

Looks like I added that in https://reviews.llvm.org/D138974 but I'm not aware of it being used anywhere at the moment. I'd still prefer renaming it to `CLANG_PGO_TRAINING_DEPS` for consistency, especially that we're now adding it to cache.

https://github.com/llvm/llvm-project/pull/78879