[ThinLTO] Make llvm-lto module ID numbering consistent with linkers

The module ID numbering typically starts at 0 (in both the new and old
LTO APIs, used by linkers). Make llvm-lto consistent with that.

Split out of D46699.

llvm-svn: 332476

Author: teresajohnson

llvm/tools/llvm-lto/llvm-lto.cpp

@@ -380,7 +380,7 @@ static void createCombinedModuleSummaryIndex() {
     ExitOnError ExitOnErr("llvm-lto: error loading file '" + Filename + "': ");
     std::unique_ptr<MemoryBuffer> MB =
         ExitOnErr(errorOrToExpected(MemoryBuffer::getFileOrSTDIN(Filename)));
-    ExitOnErr(readModuleSummaryIndex(*MB, CombinedIndex, ++NextModuleId));
+    ExitOnErr(readModuleSummaryIndex(*MB, CombinedIndex, NextModuleId++));
   }
   std::error_code EC;
   assert(!OutputFilename.empty());