Add an accessor for turbine's processor loader

which will be used to support turbine.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=267186342
diff --git a/java/com/google/turbine/binder/Processing.java b/java/com/google/turbine/binder/Processing.java
index 67ee63a..4f2fc61 100644
--- a/java/com/google/turbine/binder/Processing.java
+++ b/java/com/google/turbine/binder/Processing.java
@@ -107,7 +107,12 @@
     for (Processor processor : processorInfo.processors()) {
       processor.init(
           new TurbineProcessingEnvironment(
-              factory, filer, log, processorInfo.options(), processorInfo.sourceVersion()));
+              factory,
+              filer,
+              log,
+              processorInfo.options(),
+              processorInfo.sourceVersion(),
+              processorInfo.loader()));
     }
 
     Map<Processor, Pattern> wanted = new HashMap<>();
diff --git a/java/com/google/turbine/processing/TurbineProcessingEnvironment.java b/java/com/google/turbine/processing/TurbineProcessingEnvironment.java
index 1602fd0..f92909d 100644
--- a/java/com/google/turbine/processing/TurbineProcessingEnvironment.java
+++ b/java/com/google/turbine/processing/TurbineProcessingEnvironment.java
@@ -23,6 +23,7 @@
 import javax.annotation.processing.Messager;
 import javax.annotation.processing.ProcessingEnvironment;
 import javax.lang.model.SourceVersion;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /** Turbine's {@link ProcessingEnvironment). */
 public class TurbineProcessingEnvironment implements ProcessingEnvironment {
@@ -33,19 +34,22 @@
   private final TurbineElements turbineElements;
   private final SourceVersion sourceVersion;
   private final Messager messager;
+  private final ClassLoader processorLoader;
 
   public TurbineProcessingEnvironment(
       ModelFactory factory,
       Filer filer,
       TurbineLog log,
       Map<String, String> processorOptions,
-      SourceVersion sourceVersion) {
+      SourceVersion sourceVersion,
+      @Nullable ClassLoader processorLoader) {
     this.filer = filer;
     this.turbineTypes = new TurbineTypes(factory);
     this.processorOptions = processorOptions;
     this.sourceVersion = sourceVersion;
     this.turbineElements = new TurbineElements(factory, turbineTypes);
     this.messager = new TurbineMessager(factory, log);
+    this.processorLoader = processorLoader;
   }
 
   @Override
@@ -82,4 +86,8 @@
   public Locale getLocale() {
     return Locale.ENGLISH;
   }
+
+  public ClassLoader processorLoader() {
+    return processorLoader;
+  }
 }