[cfe-dev] Separating clang-tools-extra from clang in LLVM_ENABLE_PROJECTS

[Reid Kleckner via cfe-dev]

The context is https://reviews.llvm.org/D58157

Back when Mehdi originally introduced the LLVM_ENABLE_PROJECTS cmake
variable, people were discussing the idea of folding clang-tools-extra into
clang, if we moved to multiple git repos away from a monorepo. To support
that goal, this code was added:
https://github.com/llvm/llvm-project/blob/master/llvm/CMakeLists.txt#L144
      # There is a widely spread opinion that clang-tools-extra should be
merged
      # into clang. The following simulates it by always enabling
clang-tools-extra
      # when enabling clang.
      if (proj STREQUAL "clang")
        set(LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR
"${CMAKE_CURRENT_SOURCE_DIR}/../clang-tools-extra")
      endif()

This makes it so that if you're building clang, you're building clang-tidy,
clangd, etc. However, we have use cases where we want to check out the
monorepo and just build clang, so we wanted to remove that block.

The upshot is that if you use LLVM_ENABLE_PROJECTS today and you want to
build clang tools, you will have to add "clang-tools-extra" to your CMake
invocation after D58157 lands. We've identified the one upstream buildbot
that uses this variable and plan to update it, but if you have downstream
bots using this variable, you may also need to add "clang-tools-extra" to
build clang-tidy & co.

We have some consensus that this is the direction we want to go in the code
review from Sam Mccall, Shoaib, myself, and Nico, but please let us know if
you think this is the wrong direction.
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