[CMake] Fix `is_llvm_target_library` and support out-of-order components

Summary: This patch is required by D28855, and enables us to rely on CMake's ability to handle out of order target dependencies.

Reviewers: mgorny, chapuni, bryant

Subscribers: llvm-commits, jgosnell

Differential Revision: https://reviews.llvm.org/D28869

llvm-svn: 294514