[LTO] Export DataLayoutStr instead of the whole module

[davide]

Summary

Trying to address r281289 post-Commit review. Peter, Mehdi, is this more reasonable to you?

[mehdi_amini]

Sure. LGTM

include/llvm/LTO/LTO.h

                             symbol_iterator(Obj->symbol_end()));
   }
 
+  StringRef getDataLayoutStr() const {
+    return Obj->getModule().getDataLayoutStr();
+  }
+
   StringRef getSourceFileName() const {
     return Obj->getModule().getSourceFileName();
   }

   MemoryBufferRef getMemoryBufferRef() const {
     return Obj->getMemoryBufferRef();
   }
-
-  Module &getModule() const {
-    return Obj->getModule();
-  }
 };
 
 /// A ThinBackend defines what happens after the thin-link phase during ThinLTO.