clang-format: Improve ObjC blocks with return type.

Before:
  Block b = ^int * (A * a, B * b) {}

After:
  Block b = ^int *(A *a, B *b) {}

This fixed llvm.org/PR21619.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222639 91177308-0d34-0410-b5e6-96231b3b80d8