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

[David Spickett via cfe-commits]

DavidSpickett wrote:

I should note that llvm also has this "problem", but llvm being the first step of the chain likely you want to build llvm-tblgen anyway.

I did try applying the same kind of change there but due to the whole min-tblgen thing, it looked more complex.

There's a reasonable argument for llvm remaining "special" in this way but not anything downstream of it.

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