Add support for --javacopt flags terminated by -- to JavaBuilder

JDK 9 adds javac flags that start with `--`, so we can't assume that `--` flags
belong to JavaBuilder. Instead, we will use `--` by itself as a delimiter to parse
e.g. `--javacopts --release 8 -- sources Hello.java`.

This change just adds support for dropping a trailing `--` after parsing javacopts.
We can't rely on the delimiter until after the next Blaze release.

PiperOrigin-RevId: 179589863

src/java_tools/buildjar/java/com/google/devtools/build/buildjar/OptionsParser.java

@@ -110,6 +110,12 @@ private void processCommandlineArgs(Deque<String> argQueue) throws InvalidComman
           // terminator to the passed arguments.
           collectFlagArguments(javacOpts, argQueue, "--");
           sourcePathFromJavacOpts();
+          if (!argQueue.isEmpty() && argQueue.peekFirst().equals("--")) {
+            // Support --javacopts terminated with "--", in preparation for requiring javacopts
+            // to be terminated with "--", in preparation for supporting --javacopts that start
+            // with "--".
+            argQueue.removeFirst();
+          }
           break;
         case "--direct_dependency":
           {