[llvm] Escape LLVM_TARGETS_TO_BUILD while checking against LLVM_ALL_TARGETS and LLVM_EXPERIMENTAL_TARGETS_TO_BUILD (PR #70885)

[via llvm-commits]

natschz wrote:

> LGTM, thanks! Will you need someone to land this for you?

Yea I think so, not quite sure what to do next.

https://github.com/llvm/llvm-project/pull/70885