[llvm-exegesis] Fix compilation before c++17.

ClusteringTest.cpp:25:23: error: constexpr variable cannot have non-literal type 'const llvm::exegesis::(anonymous namespace)::(lambda at /home/buildslave/ps4-buildslave4/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/llvm.src/unittests/tools/llvm-exegesis/ClusteringTest.cpp:25:35)'
static constexpr auto HasPoints = [](const std::vector<int> &Indices) {

llvm-svn: 356748