clang-format: Revamp nested block formatting.

This fixed llvm.org/PR21804 and hopefully a few other strange cases.

Before:
    if (blah_blah(whatever, whatever, [] {
      doo_dah();
      doo_dah();
    })) {
    }
    }

After:
    if (blah_blah(whatever, whatever, [] {
          doo_dah();
          doo_dah();
        })) {
    }
    }

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224112 91177308-0d34-0410-b5e6-96231b3b80d8