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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190038 91177308-0d34-0410-b5e6-96231b3b80d8