[polly] [CMake] Create component and install target in add_polly_library (#66598)

Currently there's no component for LLVMPolly and PollyISL, however
they are added to exports whether or not they are installed. This commit
calls add_llvm_install_targets in the add_polly_library function to
allow installation of LLVMPolly and PollyISL via distribution
components, so they can be installed without also installing libPolly.a.

Closes: https://github.com/llvm/llvm-project/pull/66598