partial fix for CPP14.g4 Stringliteral concatenation

[Xyrio]

this is correct code that fails with current grammar: cout << "hello " "world";
example: https://ideone.com/aRPTIT

#include <iostream>
using namespace std;
 
int main() {
	//example a
	cout << "hello " "world" 
	"s" << endl;
	//example b
	cout << "hello " "world" \
	"s" << endl;
	return 0;
}

example a is not corrected but it should work too instead error is:
line 9:26 token recognition error at: '\\r'

i corrected example b like the following
file CPP14.g4 line 2068:
before:
| Stringliteral
after:
| Stringliteral+

[teverett]

Please submit a PR

[teverett]

PR #1328