[Frontend] Load standard libarary in CompilerInstance::setup

Instead of checking that the stdlib can be loaded in a variety of places, check it when setting up the compiler instance. This required a couple more checks to avoid loading the stdlib in cases where it’s not needed.

To be able to differentiate stdlib loading failures from other setup errors, make `CompilerInstance::setup` return an error message on failure via an inout parameter. Consume that error on the call side, replacing a previous, more generic error message, adding error handling where appropriate or ignoring the error message, depending on the context.

lib/DriverTool/swift_api_extract_main.cpp

@@ -197,8 +197,9 @@ public:
   }
 
   int extractAPI() {
-    if (Instance.setup(Invocation)) {
-      llvm::outs() << "Failed to setup compiler instance\n";
+    std::string InstanceSetupError;
+    if (Instance.setup(Invocation, InstanceSetupError)) {
+      llvm::outs() << InstanceSetupError << '\n';
       return 1;
     }