[llvm-exegesis] Fix r373083: Module -> Mod.

SnippetRepetitorTest.cpp:66:27: error: declaration of ‘std::unique_ptr<llvm::Module> llvm::exegesis::{anonymous}::X86SnippetRepetitorTest::Module’ [-fpermissive]
   std::unique_ptr<Module> Module;

llvm-svn: 373087