[Sema] Remove an unnecessary cast (NFC) (#146985)

Decl is already of FunctionDecl *.

Author: kazutakahirata

clang/lib/Sema/SemaConcept.cpp

@@ -1138,8 +1138,8 @@ bool Sema::CheckFunctionTemplateConstraints(
   }
 
   CXXThisScopeRAII ThisScope(*this, Record, ThisQuals, Record != nullptr);
-  LambdaScopeForCallOperatorInstantiationRAII LambdaScope(
-      *this, const_cast<FunctionDecl *>(Decl), *MLTAL, Scope);
+  LambdaScopeForCallOperatorInstantiationRAII LambdaScope(*this, Decl, *MLTAL,
+                                                          Scope);
 
   return CheckConstraintSatisfaction(Template, TemplateAC, *MLTAL,
                                      PointOfInstantiation, Satisfaction);