clang-format: [Proto] Improve wrapping of message field attributes.

Before:
  optional AAA aaa = 1 [foo =
			    {
			      key: "a"  //
			    },
			bar = {
			  key: "a"  //
			}];

After:
  optional AAA aaa = 1 [
    foo = {
      key: "a"  //
    },
    bar = {
      key: "a"  //
    }
  ];

llvm-svn: 256736