clang-format: Format extern "C" blocks like namespace blocks.

namespace blocks act as if KeepEmptyLinesAtTheStartOfBlocks is always true,
and aren't collapsed to a single line even if they would fit. Do the same
for extern "C" blocks.

Before,

  extern "C" {

  void ExternCFunction();

  }

was collapsed into `extern "C" { void ExternCFunction(); }`. Now it stays like
it was.

Fixes http://crbug.com/432640 and part of PR21419.

llvm-svn: 221897