[docs] Fix Kaleidoscope code example

[r7vme]

Summary

• replace virtual with override
• use default like in full code example

[r7vme]

Ready for review

[r7vme]

Hello, this is my first patch to LLVM. Let me know if anything else is needed.

[lattner]

Nice, welcome to the project!

[r7vme]

CI has passed. Could someone please merge this?

[foad]

In D123110#3430558, @r7vme wrote:

CI has passed. Could someone please merge this?

Done! I copied the author information that you use as r7vme on github. I hope that's OK.

[r7vme]

Thanks @foad :)

[lattner]

Thank you @foad! If you'd like to apply for commit access, plz follow the instructions in the llvm developer policy.

llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl03.rst

 First we define virtual code generation (codegen) methods in each AST
 class:
 
 .. code-block:: c++
 
     /// ExprAST - Base class for all expression nodes.
     class ExprAST {
     public:
-      virtual ~ExprAST() {}
+      virtual ~ExprAST() = default;
       virtual Value *codegen() = 0;
     };
 
     /// NumberExprAST - Expression class for numeric literals like "1.0".
     class NumberExprAST : public ExprAST {
       double Val;
 
     public:
       NumberExprAST(double Val) : Val(Val) {}
-      virtual Value *codegen();
+      Value *codegen() override;
     };
     ...
 
 The codegen() method says to emit IR for that AST node along with all
 the things it depends on, and they all return an LLVM Value object.
 "Value" is the class used to represent a "`Static Single Assignment
 (SSA) <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_
 register" or "SSA value" in LLVM. The most distinct aspect of SSA values