Frontend: force order of evaluation

The previous statement happened to work out of sheer luck.  `MainModule` is
initialized by the call to `getMainModule()`.  The expression could be evaluated
in any order, and in the Visual Studio case, was evaluated with the index
*first*.  At this point `MainModule` was uninitialized (fortunately, it was set
to NULL).  As a result, the `getName` call would fail.  Ensure that the
`MainModule` is initialized first by using a local variable.

Author: compnerd

lib/Frontend/Frontend.cpp

@@ -517,7 +517,9 @@ void CompilerInstance::performSemaUpTo(SourceFile::ASTStage_t LimitStage) {
   }
 
   FrontendStatsTracer tracer(Context->Stats, "perform-sema");
-  Context->LoadedModules[MainModule->getName()] = getMainModule();
+
+  ModuleDecl *mainModule = getMainModule();
+  Context->LoadedModules[mainModule->getName()] = mainModule;
 
   if (Invocation.getInputKind() == InputFileKind::SIL) {
     assert(!InputSourceCodeBufferIDs.empty());