Add 'thinlto_src_module' md only while debugging or when stats enable

[Prazek]

Summary

Fixed LTO document

[Prazek]

wtf arc y u do dis

docs/LinkTimeOptimization.rst

.. code-block:: c++
   int main() {
     return foo1();
   }
 
 To compile, run:
 
 .. code-block:: console
 
-  % clang -emit-llvm -c a.c -o a.o   # <-- a.o is LLVM bitcode file
+  % clang -flto -c a.c -o a.o        # <-- a.o is LLVM bitcode file
   % clang -c main.c -o main.o        # <-- main.o is native object file
-  % clang a.o main.o -o main         # <-- standard link command without modifications
+  % clang -flto a.o main.o -o main   # <-- standard link command without modifications
 
 * In this example, the linker recognizes that ``foo2()`` is an externally
   visible symbol defined in LLVM bitcode file. The linker completes its usual
   symbol resolution pass and finds that ``foo2()`` is not used
   anywhere. This information is used by the LLVM optimizer and it
   removes ``foo2()``.
 
 * As soon as ``foo2()`` is removed, the optimizer recognizes that condition ``i

lib/Transforms/IPO/FunctionImport.cpp

for (Function &F : *SrcModule) {
         continue;
       auto GUID = F.getGUID();
       auto Import = ImportGUIDs.count(GUID);
       DEBUG(dbgs() << (Import ? "Is" : "Not") << " importing function " << GUID
                    << " " << F.getName() << " from "
                    << SrcModule->getSourceFileName() << "\n");
       if (Import) {
         F.materialize();
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_STATS)
         // Add 'thinlto_src_module' metadata for statistics and debugging.
         F.setMetadata("thinlto_src_module",
                       llvm::MDNode::get(DestModule.getContext(),
                                         {llvm::MDString::get(
                                             DestModule.getContext(),
                                             SrcModule->getSourceFileName())}));
+#endif  // !defined(NDEBUG) || defined(LLVM_ENABLE_STATS)
         GlobalsToImport.insert(&F);
       }
     }
     for (GlobalVariable &GV : SrcModule->globals()) {
       if (!GV.hasName())
         continue;
       auto GUID = GV.getGUID();
       auto Import = ImportGUIDs.count(GUID);