[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

llvm/trunk/include/llvm/LTO/LTO.h

public:
   };
 
   /// A range over the symbols in this InputFile.
   iterator_range<symbol_iterator> symbols() {
     return llvm::make_range(symbol_iterator(Obj->symbol_begin()),
                             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.
 /// The details of this type definition aren't important; clients can only
 /// create a ThinBackend using one of the create*ThinBackend() functions below.
 typedef std::function<std::unique_ptr<ThinBackendProc>(
     Config &C, ModuleSummaryIndex &CombinedIndex,
     StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,