When we are deserializing the lexical decls of a DeclContext from PCH, notify the PCHReader to hold off passing Decls
to the consumer until the DeclContext is fully prepared.

Before, due to recursive loading, we could be in a situation where we would try to deserialize the decls of a DeclContext which was already doing that, and bad things would happen. In the specific case I encountered, the lexical decls would
form a cycle and we would enter infinite loop territory.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@109857 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/DeclBase.cpp b/lib/AST/DeclBase.cpp
index c2cab23..e4ff679 100644
--- a/lib/AST/DeclBase.cpp
+++ b/lib/AST/DeclBase.cpp
@@ -606,6 +606,9 @@
   ExternalASTSource *Source = getParentASTContext().getExternalSource();
   assert(hasExternalLexicalStorage() && Source && "No external storage?");
 
+  // Notify that we have a DeclContext that is initializing.
+  ExternalASTSource::Deserializing ADeclContext(Source);
+
   llvm::SmallVector<Decl*, 64> Decls;
   if (Source->FindExternalLexicalDecls(this, Decls))
     return;