Add some comments.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@60119 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Parse/ParseExpr.cpp b/lib/Parse/ParseExpr.cpp
index 17b14bd..66b1d18 100644
--- a/lib/Parse/ParseExpr.cpp
+++ b/lib/Parse/ParseExpr.cpp
@@ -424,7 +424,11 @@
 Parser::ExprResult Parser::ParseCastExpression(bool isUnaryExpression) {
   if (getLang().CPlusPlus) {
     // Annotate typenames and C++ scope specifiers.
-    // Used only in C++; in C let the typedef name be handled as an identifier.
+    // Used only in C++, where the typename can be considered as a functional
+    // style cast ("int(1)").
+    // In C we don't expect identifiers to be treated as typenames; if it's a
+    // typedef name, let it be handled as an identifier and
+    // Actions.ActOnIdentifierExpr will emit the proper diagnostic.
     TryAnnotateTypeOrScopeToken();
   }