Use binary semantics when __iadd__ et al are unbound (#13987)

## Summary

I noticed that augmented assignments on floats were yielding "not
supported" diagnostics. If the dunder isn't bound at all, we should use
binary operator semantics, rather than treating it as not-callable.

crates/red_knot_python_semantic/resources/mdtest/assignment/augmented.md

@@ -6,6 +6,10 @@
 x = 3
 x -= 1
 reveal_type(x)  # revealed: Literal[2]
+
+x = 1.0
+x /= 2
+reveal_type(x)  # revealed: float
 ```
 
 ## Dunder methods