Explorer: Print self pattern for callable declarations (#3216)

When printing callable declarations, it does not print the self pattern
in the deduced bindings.
Example:

User code:
```
class A {
    fn Fun[self: Self]() {}
} 
```

Print output is missing the self pattern
```
class A {
  fn Fun ()
  {
  }
}
```

This PR fixes it and includes the self pattern in the print output.
```
class A {
  fn Fun [self: Self]()
  {
  }
}
```

---------

Co-authored-by: Geoff Romer <gromer@google.com>

explorer/ast/declaration.cpp

@@ -415,12 +415,15 @@ void CallableDeclaration::PrintIndent(int indent_num_spaces,
   auto name = GetName(*this);
   CARBON_CHECK(name) << "Unexpected missing name for `" << *this << "`.";
   out.indent(indent_num_spaces) << "fn " << *name << " ";
-  if (!deduced_parameters_.empty()) {
+  if (!deduced_parameters_.empty() || self_pattern_) {
     out << "[";
     llvm::ListSeparator sep;
     for (Nonnull<const GenericBinding*> deduced : deduced_parameters_) {
       out << sep << *deduced;
     }
+    if (self_pattern_) {
+      out << sep << **self_pattern_;
+    }
     out << "]";
   }
   out << *param_pattern_;