[SDK] Rename several small overlay source files. (#4520)

...to remove the temptation to put everything in one file with the same
name as the module. This doesn't do anything for overlays that /already/
have everything in one file with the same name as the module, except for
a few easy cases; we can unpack the rest later.

stdlib/public/SDK/CoreData/NSManagedObjectContext.swift

@@ -0,0 +1,12 @@
+@_exported import CoreData
+import Foundation
+
+extension NSManagedObjectContext {
+  public func fetch<T: NSFetchRequestResult>(_ request: NSFetchRequest<T>) throws -> [T] {
+    return try fetch(unsafeBitCast(request, to: NSFetchRequest<NSFetchRequestResult>.self)) as! [T]
+  }
+
+  public func count<T: NSFetchRequestResult>(for request: NSFetchRequest<T>) throws -> Int {
+    return try count(for: unsafeBitCast(request, to: NSFetchRequest<NSFetchRequestResult>.self))
+  }
+}