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

[Ross Burton via cfe-commits]

rossburton wrote:

So my approach was wrong. My goal is to not build clang-tblgen if it's already been provided, but not explode the build if it hasn't. Obviously I'm doing something wrong with this approach and either I need to fiddle the logic in `add_tablegen` or the behaviour I'm after already exists if I pass the right arguments.

Currently, if I do a non-cross build and set `CLANG_TABLEGEN_EXE`, it builds a tablegen binary that is not used nor installed.

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