[cmake] Don't install static extensions when linked into libLLVM

[xarblu]

When extensions like polly are linked as part of libLLVM these don't need to be and likely shouldn't be installed.

Reason being that e.g. out-of-tree clang builds against libLLVM will try to statically link these extensions
when calling process_llvm_pass_plugins.

1. This is undesirable because the whole point is to have a shared library to link to
2. It actually causes issues because LLVMExports.cmake will include e.g. libPollyISL, LLVMPolly, etc. as installed targets depending on on non-installed LLVM*.a targets which are available via libLLVM.so

This is mainly intended to allow distributions to ship polly enabled libLLVM while still keeping e.g. clang independent and linked dynamically.

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[xarblu]

Nevermind, this still doesn't work the way I expected. While it does allow clang to fully dynamically link to a polly-enabled libLLVM clang will then hard-depend on polly symbols in libLLVM and break when libLLVM is build with polly disabled.