[test] -Fsystem $SDK/System/Library/PrivateFrameworks (#23019)

Otherwise certain modules don't import correctly when built against an
Apple-internal SDK. Should have no effect when using the public SDK,
but we try to minimize divergence between the public and
Apple-internal test suites.

rdar://problem/48513002

validation-test/SIL/verify_all_overlays.py

@@ -1,6 +1,7 @@
 #!/usr/bin/python
 # RUN: ${python} %s %target-swiftmodule-name %platform-sdk-overlay-dir \
 # RUN:     %target-sil-opt -sdk %sdk -enable-sil-verify-all \
+# RUN:       -Fsystem %sdk/System/Library/PrivateFrameworks \
 # RUN:       -F "%xcode-extra-frameworks-dir"
 
 # REQUIRES: long_test