[Polly][CMake] Fix lit setup for building the in the mono repo

Summary:
When building polly as part of the monorepo (actually, as part of any setup
using LLVM_ENABLE_PROJECTS), the LLVMPolly library used in the lit tests ends
up in a different directory in the build tree than in an in-tree build

Reviewers: Meinersbur, grosser, bollu

Reviewed By: Meinersbur

Subscribers: mgorny, bollu, pollydev, llvm-commits

Differential Revision: https://reviews.llvm.org/D44078

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@326702 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed