clang-format: Fix segfault in 'incomplete' macros.

The code leading to a segfault was:
  #pragma omp threadprivate(y)), // long comment leading to a line break

This fixes llvm.org/PR16513.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189460 91177308-0d34-0410-b5e6-96231b3b80d8