Add a const version of Expr::IgnoreImplicit. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@160801 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang/AST/Expr.h b/include/clang/AST/Expr.h
index d9cf0ad..bb66e71 100644
--- a/include/clang/AST/Expr.h
+++ b/include/clang/AST/Expr.h
@@ -596,6 +596,10 @@
     return cast<Expr>(Stmt::IgnoreImplicit());
   }
 
+  const Expr *IgnoreImplicit() const LLVM_READONLY {
+    return const_cast<Expr*>(this)->IgnoreImplicit();
+  }
+
   /// IgnoreParens - Ignore parentheses.  If this Expr is a ParenExpr, return
   ///  its subexpression.  If that subexpression is also a ParenExpr,
   ///  then this method recursively returns its subexpression, and so forth.