[SDK] Fix circularity issues with overlay shim headers.

Loading a Clang module eagerly brings in overlays for anything it re-exports,
but this is a problem for these new "shim header" modules, which generally
import the underlying module for an overlay and are in turn imported by the
overlay. That means that when we try to import an overlay, we'll end up with
a circular reference before it's done loading all its dependencies. Break the
cycle by not exporting anything from these modules, which are mostly just an
implementation detail anyway.

Author: jrose-apple

stdlib/public/SwiftShims/module.modulemap

@@ -17,27 +17,29 @@ module SwiftShims {
   export *
 }
 
+// Various headers used to build overlays on Apple platforms.
+// Note: These deliberately do not use "export *" to avoid circularity issues
+// (overlay -> shims -> framework -> overlay)
+// FIXME: These are only needed when building each overlay; they declare no
+// types and therefore would not strictly need to be present in an installed
+// Swift.
+// FIXME: These are not used at all on non-Apple platforms.
 module _SwiftObjectiveCOverlayShims {
   header "ObjectiveCOverlayShims.h"
-  export *
 }
 
 module _SwiftOSOverlayShims {
   header "OSOverlayShims.h"
-  export *
 }
 
 module _SwiftSafariServicesOverlayShims {
   header "SafariServicesOverlayShims.h"
-  export *
 }
 
 module _SwiftXCTestOverlayShims {
   header "XCTestOverlayShims.h"
-  export *
 }
 
 module _SwiftXPCOverlayShims {
   header "XPCOverlayShims.h"
-  export *
 }