Fix up compnerd's change 46d684f to stop using BitstreamReader.

The more important fix here is to remove a duplicated call to advance the
cursor, which led to module import being completely broken. I also added
back the AF_DontPopBlockAtEnd flags that were dropped. I am not certain
if they are needed but without clear indication otherwise, we should keep
them.

Author: bob-wilson

lib/Serialization/ModuleFile.cpp

@@ -242,7 +242,7 @@ ValidationInfo serialization::validateSerializedAST(
   llvm::BitstreamEntry topLevelEntry;
 
   while (!cursor.AtEndOfStream()) {
-    topLevelEntry = cursor.advance();
+    topLevelEntry = cursor.advance(AF_DontPopBlockAtEnd);
     if (topLevelEntry.Kind != llvm::BitstreamEntry::SubBlock)
       break;
 
@@ -774,7 +774,7 @@ ModuleFile::ModuleFile(
   llvm::BitstreamEntry topLevelEntry;
 
   while (!cursor.AtEndOfStream()) {
-    topLevelEntry = cursor.advance();
+    topLevelEntry = cursor.advance(AF_DontPopBlockAtEnd);
     if (topLevelEntry.Kind != llvm::BitstreamEntry::SubBlock)
       break;
 
@@ -973,8 +973,6 @@ ModuleFile::ModuleFile(
       }
       break;
     }
-
-    topLevelEntry = cursor.advance(AF_DontPopBlockAtEnd);
   }
 
   if (topLevelEntry.Kind != llvm::BitstreamEntry::EndBlock) {
@@ -993,7 +991,7 @@ ModuleFile::ModuleFile(
   }
 
   while (!docCursor.AtEndOfStream()) {
-    topLevelEntry = docCursor.advance();
+    topLevelEntry = docCursor.advance(AF_DontPopBlockAtEnd);
     if (topLevelEntry.Kind != llvm::BitstreamEntry::SubBlock)
       break;