clang-format: Correctly mark preprocessor lines in child blocks.

This prevents contracting:
  auto lambda = []() {
    int a = 2
  #if A
            + 2
  #endif
        ;
  };

into:
  auto lambda = []() { int a = 2
  #if A + 2
  #endif ; };

Which is obviously BAD.

This fixes llvm.org/PR22496.

llvm-svn: 228522