[cmake] Optionally install clang-tblgen to aid cross-compiling

clang-tblgen is required to cross-compile clang itself. Unlike before,
most of the infrastructure is in place to do this now, and the only
thing preventing it is LLVM_BUILD_UTILS, which doesn't apply to Clang.

I thought about changing this to ${project}_BUILD_UTILS and adding
a CLANG_BUILD_UTILS option, but there seems little point for just one tool.

Instead, it checks whether clang-tblgen was explicitly requested in
LLVM_DISTRIBUTION_COMPONENTS, which is good enough for Gentoo and
other distributions.

Closes https://github.com/llvm/llvm-project/issues/20282.

Differential Revision: https://reviews.llvm.org/D141092