[CMake] lli/CMakeLists.txt: Move add_subdirectory(ChildTarget) to the front. It depends on nothing described in LLVM_LINK_COMPONENTS.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196902 91177308-0d34-0410-b5e6-96231b3b80d8