clang/Modules: Add missing diagnostics for malformed AST files

[dexonsmith]

Summary

These were found via an audit. In the case of ParseLineTable this is
actually dead code, since parsing the line table always succeeds, but
it's prudent to be defensive since it's possible an assertion there
could be converted to a true return in the future.

[aprantl]

clang/lib/Serialization/ASTReader.cpp

3412

Note for self, this is not an llvm::Error object but ASTReader::Error().

[dexonsmith]

Committed 8e2c192e2af8c760152ba3b28e774dbb1548e4aa.

clang/lib/Serialization/ASTReader.cpp

case SOURCE_LOCATION_OFFSETS: {
       break;
     }
 
     case MODULE_OFFSET_MAP:
       F.ModuleOffsetMap = Blob;
       break;
 
     case SOURCE_MANAGER_LINE_TABLE:
-      if (ParseLineTable(F, Record))
+      if (ParseLineTable(F, Record)) {
+        Error("malformed SOURCE_MANAGER_LINE_TABLE in AST file");
         return Failure;
+      }
       break;
 
     case SOURCE_LOCATION_PRELOADS: {
       // Need to transform from the local view (1-based IDs) to the global view,
       // which is based off F.SLocEntryBaseID.
       if (!F.PreloadSLocEntries.empty()) {
         Error("Multiple SOURCE_LOCATION_PRELOADS records in AST file");
         return Failure;
Expected<unsigned> MaybeRecCode =
         Stream.readRecord(Entry.ID, Record, &Blob);
     if (!MaybeRecCode) {
       Error(MaybeRecCode.takeError());
       return Failure;
     }
     switch (MaybeRecCode.get()) {
     case EXTENSION_METADATA: {
       ModuleFileExtensionMetadata Metadata;
-      if (parseModuleFileExtensionMetadata(Record, Blob, Metadata))
+      if (parseModuleFileExtensionMetadata(Record, Blob, Metadata)) {
+        Error("malformed EXTENSION_METADATA in AST file");
         return Failure;
+      }
 
       // Find a module file extension with this block name.
       auto Known = ModuleFileExtensions.find(Metadata.BlockName);
       if (Known == ModuleFileExtensions.end()) break;
 
       // Form a reader.
       if (auto Reader = Known->second->createExtensionReader(Metadata, *this,
                                                              F, Stream)) {