Module trace: explicitly ignore the main module

I'm not sure why this wasn't already broken, but with the changes in
the next commit the multifile module trace test fails complaining that
it can't get a path for the main module. But we don't /need/ a path
for the main module because it's not a dependency of itself.

Author: jrose-apple

lib/FrontendTool/FrontendTool.cpp

@@ -268,6 +268,8 @@ static bool emitLoadedModuleTraceIfNeeded(ModuleDecl *mainModule,
   for (auto &module : ctxt.LoadedModules) {
     ModuleDecl *loadedDecl = module.second;
     assert(loadedDecl && "Expected loaded module to be non-null.");
+    if (loadedDecl == mainModule)
+      continue;
     assert(!loadedDecl->getModuleFilename().empty()
            && "Don't know how to handle modules with empty names.");
     pathToModuleDecl.insert(