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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294514 91177308-0d34-0410-b5e6-96231b3b80d8