Currently during module importing, ThinLTO opens all the source modules, collect functions to be imported and append them to the destination module, then leave all the modules open through out the lto backend pipeline. This patch refactors it in the way that one source module will be closed before another source module is opened. All the source modules will be closed after importing phase is done. It will save some amount of memory when there are many source modules to be imported.
Note that this patch only changes the distributed thinlto mode. For in process thinlto mode, one source module is shared acorss different thinlto backend threads so it is not changed in this patch.