clang-format: Enable formatting of nested blocks.

Among other things, this enables (better) formatting lambdas and
constructs like:
  MACRO({
          long_statement();
          long_statement_2();
        },
        {
          long_statement();
          long_statement_2();
        },
        { short_statement(); }, "");

This fixes llvm.org/PR15381.

llvm-svn: 190038