Fix GSYM tests to run the yaml files and fix test failures on some machines.

Summary: YAML files were not being run during lit testing as there was no lit.local.cfg file. Once this was fixed, some buildbots would fail due to a StringRef that pointed to a std::string inside of a temporary llvm::Triple object. These issues are fixed here by making a local triple object that stays around long enough so the StringRef points to valid data.

Reviewers: aprantl, thakis, MaskRay, aadsm, wallace

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75390