[ASTDump] Add a flag indicating whether a CXXThisExpr is implicit

There is currently no way to distinguish implicit from explicit
CXXThisExpr in the AST dump output.

Differential Revision: https://reviews.llvm.org/D57649

Reviewed By: steveire

llvm-svn: 353003
diff --git a/clang/lib/AST/TextNodeDumper.cpp b/clang/lib/AST/TextNodeDumper.cpp
index 6e6700c..9e6f721 100644
--- a/clang/lib/AST/TextNodeDumper.cpp
+++ b/clang/lib/AST/TextNodeDumper.cpp
@@ -854,6 +854,8 @@
 }
 
 void TextNodeDumper::VisitCXXThisExpr(const CXXThisExpr *Node) {
+  if (Node->isImplicit())
+    OS << " implicit";
   OS << " this";
 }