clang-format: Operator precendence in ObjC method exprs.

Patch (mostly) by Adam Strzelecki. Thanks!

Before:
  [self aaaaaa:bbbbbbbbbbbbb
      aaaaaaaaaa:bbbbbbbbbbbbbbbbb
           aaaaa:bbbbbbbbbbb +
      bbbbbbbbbbbb aaaa:bbb];

After:
  [self aaaaaa:bbbbbbbbbbbbb
      aaaaaaaaaa:bbbbbbbbbbbbbbbbb
           aaaaa:bbbbbbbbbbb + bbbbbbbbbbbb
            aaaa:bbb];

This fixes llvm.org/PR16150.