[AST] NFC: Pretty stack trace for Traversal::visitNominalTypeDecl

This helps a bit in getting more readable stack traces when the compiler crashes.

Author: egorzhdan

lib/AST/ASTWalker.cpp

@@ -284,6 +284,9 @@ class Traversal : public ASTVisitor<Traversal, Expr*, Stmt*,
   }
 
   bool visitNominalTypeDecl(NominalTypeDecl *NTD) {
+#ifndef NDEBUG
+    PrettyStackTraceDecl debugStack("walking into", NTD);
+#endif
 
     bool WalkGenerics = visitGenericParamListIfNeeded(NTD);