[CMake] Avoid liblldb genex when figuring out the copy destination for framework tools

This genex created an order-only dependency to liblldb for every framework tool. It reduced build throughput in the first half of the compilation and pulled in unnecessary build units, e.g. debugserver required ~900 build units. With this change debugserver is (again) down at 52 build units!

llvm-svn: 366350