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

[David Spickett via cfe-commits]

DavidSpickett wrote:

Found a similar situation:
https://github.com/llvm/llvm-project/blob/0db5ba0ce6c36c78f93ebfe992c0f9ed807328bb/llvm/CMakeLists.txt#L1348

Not allowed to disable llvm utils and include llvm tests. Though in this case, 99% of tests would not run, rather than the maybe 1% you need clang-tblgen for.

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