[Polly] Reuse LLVM's build rules for gtest/gmock

Reuse LLVM's CMakeLists.txt for gtest/gmock instead of reinventing
them in Polly.  This fixes a lot of linking errors due to not linking
LLVMSupport in for me.

Differential Revision: https://reviews.llvm.org/D85280

GitOrigin-RevId: c8b629a3a5854506dd25e84f9b4174b53f8f667b
1 file changed