cmake: Add polly libraries to LLVMExports

This fixes LINK_POLLY_INTO_TOOLS=ON builds, which previously failed with:

CMake Error: install(EXPORT "LLVMExports" ...) includes target "opt" which
requires target "Polly" that is not in the export set.

CMake Error: install(EXPORT "LLVMExports" ...) includes target "bugpoint" which
requires target "Polly" that is not in the export set.