[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/Frontend/ModuleInterfaceBuilder.cpp

@@ -218,13 +218,6 @@ bool ModuleInterfaceBuilder::buildSwiftModuleInternal(
     InputInfo.getPrimarySpecificPaths().SupplementaryOutputs;
     StringRef OutPath = OutputInfo.ModuleOutputPath;
 
-    // Bail out if we're going to use the standard library but can't load it. If
-    // we don't do this before we try to build the interface, we could end up
-    // trying to rebuild a broken standard library dozens of times due to
-    // multiple calls to `ASTContext::getStdlibModule()`.
-    if (SubInstance.loadStdlibIfNeeded())
-      return std::make_error_code(std::errc::not_supported);
-
     // Build the .swiftmodule; this is a _very_ abridged version of the logic
     // in performCompile in libFrontendTool, specialized, to just the one
     // module-serialization task we're trying to do here.