[polly][unittests] Link DeLICMTests with libLLVMCore

A `-DBUILD_SHARED_LIBS=ON` build on Solaris/amd64 failed with

  Undefined                       first referenced
   symbol                             in file
  _ZNK4llvm3cfg6UpdateIPNS_10BasicBlockEE4dumpEv tools/polly/unittests/DeLICM/CMakeFiles/DeLICMTests.dir/DeLICMTest.cpp.o  (symbol belongs to implicit dependency /var/llvm/local-amd64-release-stage2-shared-A/bin/../lib/libLLVMCore.so.14git)
  ld: fatal: symbol referencing errors

Solaris `ld` requires to directly link with dependant libraries, so this
patch explicitly adds `libLLVMCore`.

Tested on `amd64-pc-solaris2.11` and `x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D118452