[CMake] Adjust export_executable_symbols to cope with non-target link libraries

[CMake] Adjust export_executable_symbols to cope with non-target link libraries

export_executable_symbols looks though the link libraries of the executable in
order to figure out transitive dependencies, but in doing so it assumes that
all link libraries are also targets. This is not true as of r273302, so adjust
it to check if they actually are targets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274546 91177308-0d34-0410-b5e6-96231b3b80d8