clang-format: Properly reset nested AnnotatedLine structure.

This fixes llvm.org/PR17682.

Without this patch, the following code leads to invalid reads/writes:
  DEBUG({
    return aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa +
           aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa;
  });
  #if a
  #else
  #endif

Because of the #if-#else structure, the code is formatted and annotated
twice and becauce of the nested block, the annotated lines form a
hierarchical structure. This structure was not properly reset between
runs.