clang-format: Better support for CUDA's triple brackets.

Before:
  aaaaaaaaaaaaaaa<
      aaaaaaaaa, aaaaaaaaaa,
      aaaaaaaaaaaaaa><<<aaaaaaaaa, aaaaaaaaaa, aaaaaaaaaaaaaaaaaa>>>();

After:
  aaaaaaaaaaaaaaa<aaaaaaaaa, aaaaaaaaaa, aaaaaaaaaaaaaa>
      <<<aaaaaaaaa, aaaaaaaaaa, aaaaaaaaaaaaaaaaaa>>>();

llvm-svn: 286041