IRGen: simplify the stdlib special casing for Windows
Remove `IRGenModule::useDllStorage()` as there is a standalone version
that is available and the necessary information is public from the
`IRGenModule` type. Additionally, avoid the wrapped `isStandardLibrary`
preferring to use the same method off of the public accessors. This
works towards removing some of the standard library special casing so
that it is possible to support both static and dynamic standard
libraries on Windows.
