[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332476 91177308-0d34-0410-b5e6-96231b3b80d8

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());