[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/MapKit/NSValue.swift.gyb

@@ -0,0 +1,8 @@
+@_exported import MapKit // Clang module
+import Foundation
+
+%{
+from gyb_foundation_support import ObjectiveCBridgeableImplementationForNSValue
+}%
+
+${ ObjectiveCBridgeableImplementationForNSValue("MKCoordinateSpan") }