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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I don't have sufficient expertise with cmake to know whether this is a good design or not. I think the logic makes sense of only building tablegen if it's actually needed, but I'm not certain what the fallout from these changes will be. I've added @petrhosek for some more cmake expertise, but perhaps we need build manager eyeballs as well?

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