[Polly][CMake] Make PollyConfig.cmake relocatable like other projects'

[Ericson2314]

Summary

This builds on D116521 to add this functionality to Polly without more copy-pasted code.

[Ericson2314]

polly/CMakeLists.txt

72

This duplicated the above so I just deleted it.

[beanz]

LGTM

[Ericson2314]

Rebase

[Ericson2314]

This is actually *not* working, CI reports. I haven't yet figured out why the same idiom that works everywhere else doesn't work here. Back to the drawing board.

[Ericson2314]

Fix relatie path calculation issue in exports.

Instead reuse the same shared logic to get the path and do it that way.

[Ericson2314]

polly/cmake/CMakeLists.txt

101–123

This is the new bit, replacing the wold way of doing relative exports.

Someday, it would be nice to replace this the usual LLVMDistributionSupport methods, but that is a bigger change best left for later.

[Ericson2314]

polly/cmake/CMakeLists.txt

101–123

Not sure where @beanz is at, and this was a just a small addition to the approved part, so I am just going go forward with it.