[clang][ExprConst] Remove unnecessary cast

FD is a FunctionDecl, so no need to cast a FunctionDecl to a
CXXMethodDecl just to assign it to a FunctionDecl.

clang/lib/AST/ExprConstant.cpp

@@ -8006,7 +8006,8 @@ class ExprEvaluatorBase
           assert(CorrespondingCallOpSpecialization &&
                  "We must always have a function call operator specialization "
                  "that corresponds to our static invoker specialization");
-          FD = cast<CXXMethodDecl>(CorrespondingCallOpSpecialization);
+          assert(isa<CXXMethodDecl>(CorrespondingCallOpSpecialization));
+          FD = CorrespondingCallOpSpecialization;
         } else
           FD = LambdaCallOp;
       } else if (FD->isReplaceableGlobalAllocationFunction()) {