refactor: don't forward -1 as the diagnosticCode (#16495)

## Description

This change ensures that if the diagnostic code is set to -1 that we
instead forward None as the code. This will hopefully help any tooling
consuming this to not have to add checks for -1 and filter that code
out, since it's a useless code.

For example I just hit on this for the first time and saw this:

<img width="834" alt="Screenshot 2022-12-10 at 11 48 44"
src="https://user-images.githubusercontent.com/13974112/206852283-07c2fa6b-1ad2-4586-860d-d958e4db88ea.png">

Ideally we don't this showing, but if the code is forwarded to the
client via LSP like it is in the image, the client has no way to know if
-1 is a valid code or not.

Author: prolativ

sbt-bridge/src/dotty/tools/xsbt/Problem.java

@@ -41,12 +41,19 @@ public Optional<String> rendered() {
   }
 
   public Optional<xsbti.DiagnosticCode> diagnosticCode() {
-    // NOTE: It's important for compatibility that we only construct a
-    // DiagnosticCode here to maintain compatibility with older versions of
-    // zinc while using this newer version of the compiler. If we would
-    // contstruct it earlier, you'd end up with ClassNotFoundExceptions for
-    // DiagnosticCode.
-    return Optional.of(new DiagnosticCode(_diagnosticCode, Optional.empty()));
+    // We don't forward the code if it's -1 since some tools will assume that this is actually
+    // the diagnostic code and show it or attempt to use it. This will ensure tools consuming
+    // this don't all have to be adding checks for -1.
+    if (_diagnosticCode == "-1") {
+      return Optional.empty();
+    } else {
+      // NOTE: It's important for compatibility that we only construct a
+      // DiagnosticCode here to maintain compatibility with older versions of
+      // zinc while using this newer version of the compiler. If we would
+      // contstruct it earlier, you'd end up with ClassNotFoundExceptions for
+      // DiagnosticCode.
+      return Optional.of(new DiagnosticCode(_diagnosticCode, Optional.empty()));
+    }
   }
 
   @Override