GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/RecordInfo.h

Index: tools/clang/blink_gc_plugin/RecordInfo.h
diff --git a/tools/clang/blink_gc_plugin/RecordInfo.h b/tools/clang/blink_gc_plugin/RecordInfo.h
index 39c13f01c6dd9dca7134df1787854fd8b7e27a41..e672282f4f00f365cd8a95473e9dad459ef69b23 100644
--- a/tools/clang/blink_gc_plugin/RecordInfo.h
+++ b/tools/clang/blink_gc_plugin/RecordInfo.h
@@ -16,6 +16,7 @@
 
 #include "clang/AST/AST.h"
 #include "clang/AST/CXXInheritance.h"
+#include "clang/Frontend/CompilerInstance.h"
 
 class RecordCache;
 
@@ -26,6 +27,7 @@ class GraphPoint {
   virtual ~GraphPoint() {}
   void MarkTraced() { traced_ = true; }
   bool IsProperlyTraced() { return traced_ || !NeedsTracing().IsNeeded(); }
+  bool IsInproperlyTraced() { return traced_ && NeedsTracing().IsIllegal(); }
   virtual const TracingStatus NeedsTracing() = 0;
 
  private:
@@ -158,6 +160,11 @@ class RecordInfo {
 
 class RecordCache {
  public:
+  RecordCache(clang::CompilerInstance& instance)
+    : instance_(instance)
+  {
+  }
+
   RecordInfo* Lookup(clang::CXXRecordDecl* record);
 
   RecordInfo* Lookup(const clang::CXXRecordDecl* record) {
@@ -188,7 +195,11 @@ class RecordCache {
     }
   }
 
+  clang::CompilerInstance& instance() const { return instance_; }
+
  private:
+  clang::CompilerInstance& instance_;
+
   typedef std::map<clang::CXXRecordDecl*, RecordInfo> Cache;
   Cache cache_;
 };