Fix ASTReader invalid bounds check

[christylee]

Summary

The ASTReader does the index bounds check before incrementing the index, so Clang :: Modules/module_file_info.m failed on include/llvm/ADT/SmallVector.h:153 assert(idx < size()). This fixes the bounds check by incrementing first.

[smeenai]

Is there any way to add a test for this, to prevent future regressions?

[christylee]

include/llvm/ADT/SmallVector.h:153 has an assert statement, but I'm not sure why I'm the only one who's experienced problems so far.

clang/lib/Serialization/ASTReader.cpp

case INPUT_FILE_OFFSETS: {
       }
       break;
     }
 
     case IMPORTS: {
       if (!NeedsImports)
         break;
 
-      unsigned Idx = 0, N = Record.size();
-      while (Idx < N) {
+      // Kind, ImportLoc, Size, ModTime, Signature
+      unsigned chunkSize = 1+1+1+1+5;
+      unsigned N = Record.size();
+      for (unsigned Idx = chunkSize; Idx < N; Idx += chunkSize) {
         // Read information about the AST file.
-        Idx += 1+1+1+1+5; // Kind, ImportLoc, Size, ModTime, Signature
         std::string ModuleName = ReadString(Record, Idx);
         std::string Filename = ReadString(Record, Idx);
         ResolveImportedPath(Filename, ModuleDir);
         Listener.visitImport(ModuleName, Filename);
       }
       break;
     }