[test-suite] Add GCC C Torture Suite
This patch adds support for testing clang/LLVM against the GCC Torture
This patch adds the CMake configuration and licence information for these tests. A follow-up patch will add the testcases themselves (which are too large to review, and included without modifications). They will be committed together.
Reviewers: hfinkel, kristof.beyls, asb
Reviewed By: kristof.beyls
Subscribers: khcheang, mehdi_amini, jvesely, krytarowski, fedor.sergeev, zzheng, steven_wu, dexonsmith, arphaman, jfb, mstorsjo, lewis-revill, simoncook, s.egerton, riccibruno, asb, mgorny, llvm-commits
Differential Revision: https://reviews.llvm.org/D66887