Dynamically-Scoped Attributes generate error(33): missing code generation template SetNonLocalAttrHeader ONLY in C++ target.

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Target: c++
Generating parser code with ANTLR 4.13.0
Expected behavior: does not trigger any error .
Actual behavior: triggers an `error(33): missing code generation template SetNonLocalAttrHeader`

The DynScope grammar from the doc triggers the error. (from "Actions and Attributes" chapter, exactly as it is in it.)

`antlr4 -Dlanguage=Java DynScope.g4` works

`antlr4 -Dlanguage=PHP DynScope.g4` works

`antlr4 -Dlanguage=JavaScript DynScope.g4` works

`antlr4 -Dlanguage=Cpp DynScope.g4` does not work and generate the error(33).

Here is a simplified version of the grammar that triggers the same error:
```
grammar DynScope;

prog: block ;

block locals [int myInt = 0;]
: '{' decl* '}'
;

decl:
'int' INT {$block::myInt += 1;} ';'
;

INT : [0-9]+ ;
WS : [ \t\r\n]+ -> skip ;
```
if i just comment the `{$block::myInt+=1;} ';'`, no errors is triggered.
like that:
```
grammar DynScope;

prog: block ;

block locals [int myInt = 0;]
: '{' decl* '}'
;

decl:
'int' INT // {$block::myInt += 1;} ';'
;

INT : [0-9]+ ;
WS : [ \t\r\n]+ -> skip ;
```
replacing `{$block::myInt += 1;}` by `{((BlockContext*)$ctx->parent)->myInt +=1 ;}` seems to work properly.

Start with the DynScope grammar from the “Actions and Attributes” documentation and reproduce the failure using `antlr4 -Dlanguage=Cpp DynScope.g4`; compare it with the Java, PHP, and JavaScript generations that succeed. Done means C++ generation no longer reports the missing `SetNonLocalAttrHeader` template for dynamically scoped attributes.

Written by the indexing model from the issue text.