[Concurrency] Remove -executor-factory option and replace with magic type.

We decided that using a magic typealias to set the executor factory was better
than using a compiler option. Remove the `-executor-factory` option, and replace
by looking up the `DefaultExecutorFactory` type, first in the main module, and
then if that fails in Concurrency.

rdar://149058236

include/swift/Basic/LangOptions.h

@@ -409,10 +409,6 @@ namespace swift {
     /// Specifies how strict concurrency checking will be.
     StrictConcurrency StrictConcurrencyLevel = StrictConcurrency::Minimal;
 
-    /// Specifies the name of the executor factory to use to create the
-    /// default executors for Swift Concurrency.
-    std::optional<std::string> ExecutorFactory;
-
     /// Enable experimental concurrency model.
     bool EnableExperimentalConcurrency = false;