[clang-tools-extra] [cmake] Link against libclang-cpp whenever possible

Use clang_target_link_libraries() in order to support linking against
libclang-cpp instead of static libraries.

Differential Revision: https://reviews.llvm.org/D68448

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@373786 91177308-0d34-0410-b5e6-96231b3b80d8
27 files changed