[clang] [clang] Only build clang-tblgen if it is actually needed (PR #161952)

[David Spickett via cfe-commits]

DavidSpickett wrote:

I will recuse myself from reviewing since I had input to the patch already, but want to highlight this that I said earlier:

> In theory someone could be relying on there being another clang-tblgen in bin, maybe for installs, but that seems like a thing we can afford to break. They were building clang-tblgen anyway, so they could stop setting CLANG_TABLEGEN_EXE, and use the newly built one instead.

This is the main risk I see with doing this, and my guess of its impact may be totally wrong.

@rossburton could you add to the PR description the justification for the change, and the use case you want it for? Which I think is roughly - don't build clang-tblgen twice - it'll be used in some sort of build chain in Yocto.

https://github.com/llvm/llvm-project/pull/161952