clang-format: Understand single-line comments at the end of blocks.

clang-format: Understand single-line comments at the end of blocks.

This prevents clang-format from moving/aligning the comment in the
snippet:
  void f() {
    int i; // some comment
    // some unrelated comment
  }

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@225352 91177308-0d34-0410-b5e6-96231b3b80d8