C++ runtime version of previousTokenOnChannel doesn't handle edge case correctly #2343

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As documented previousTokenOnChannel, should "return -1 if there are no tokens on channel between i and 0."  but the C++ version of the runtime returns 0 not -1 as can be seen below:

```
while (true) {
    Token *token = _tokens[i].get();
    if (token->getType() == Token::EOF || token->getChannel() == channel) {
      return i;
    }

    if (i == 0)
      break;
    i--;
  }
```

Looking at the Java implementation, it would seem the C++ code should instead be:
```
while (true) {
    Token *token = _tokens[i].get();
    if (token->getType() == Token::EOF || token->getChannel() == channel) {
      return i;
    }

    if (i == 0)
      return -1;
    i--;
  }
```
This bug causes getHiddenTokensToLeft() to miss hidden tokens that come before the first non-hidden token.  There would also be a potential bug in CommonTokenStream::LB as the "< 0" case would never happen.

runtime/Cpp/runtime/src/BufferedTokenStream.cpp

@@ -272,7 +272,7 @@ ssize_t BufferedTokenStream::previousTokenOnChannel(size_t i, size_t channel) {
     }
 
     if (i == 0)
-      break;
+      return -1;
     i--;
   }
   return i;