Fix FunctionImport export list computation: need to take a reference to a map entry to actually modify it

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266159
diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index 63cfca4..088b114 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -177,7 +177,7 @@
     // Make exports in the source module.
     auto ExportModulePath = CalleeSummary->modulePath();
     if (ExportLists) {
-      auto ExportList = (*ExportLists)[ExportModulePath];
+      auto &ExportList = (*ExportLists)[ExportModulePath];
       ExportList.insert(GUID);
       // Mark all functions and globals referenced by this function as exported
       // to the outside if they are defined in the same source module.