[CMake] Factor out config prefix finding logic

See the docs in the new function for details.

 I think I found every instance of this copy pasted code. Polly could
 also use it, but currently does something different, so I will save the
 behavior change for a future revision.

We get the shared, non-installed CMake modules following the pattern
established in D116472.

It might be good to have LLD and Flang also use this, but that would be
a functional change and so I leave it as future work.

Reviewed By: beanz, lebedev.ri

Differential Revision: https://reviews.llvm.org/D116521