[polly] move detection of polly libs to LLVM

[Sebastian Pop]

Chandler Carruth wrote:
> Why is this needed? I would really rather these stay isolated in polly...

We need this to statically link polly into tools like opt.

If we only link LLVMPolly.a against the libs required by polly, linking
LLVMPolly.a into the tools, the linker complains about missing symbols.
This does not happen when linking into the tools both LLVMPolly.a and the
required polly libs.

Sebastian
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