[Compiler plugins] Fix missing return along error-handling paths

Fixes a crash when we cannot load a compiler plugin for some reason,
tracked by rdar://107797992.

Author: DougGregor

lib/AST/ASTContext.cpp

@@ -6429,6 +6429,7 @@ LoadedLibraryPlugin *ASTContext::loadLibraryPlugin(StringRef path) {
   if (!plugin) {
     Diags.diagnose(SourceLoc(), diag::compiler_plugin_not_loaded, path,
                    llvm::toString(plugin.takeError()));
+    return nullptr;
   }
 
   return plugin.get();