Changes

[test-suite] Add GCC C Torture Suite
Summary: This patch adds support for testing clang/LLVM against the GCC
Torture suite.
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
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D66887
llvm-svn: 374155