chore: add a shortcut Module ℂ ℂ instance for computability (leanprover-community#29609)

This makes the construction in [#new members > Defining an inner product to use Cauchy-Schwarz @ 💬](https://leanprover.zulipchat.com/#narrow/channel/113489-new-members/topic/Defining.20an.20inner.20product.20to.20use.20Cauchy-Schwarz/near/539179544) computable.

Previously to make it computable, it could not be defined in the point-free way.

Author: eric-wieser

Mathlib/Analysis/Complex/Basic.lean

@@ -40,6 +40,10 @@ We also register the fact that `ℂ` is an `RCLike` field.
 
 assert_not_exists Absorbs
 
+/-- A shortcut instance to ensure computability; otherwise we get the noncomputable instance
+`Complex.instNormedField.toNormedModule.toModule`. -/
+instance Complex.instModuleSelf : Module ℂ ℂ := delta% inferInstance
+
 noncomputable section
 
 namespace Complex