Sema: Add test guarding against accidental SPI reexport via clang modules

Exported imports are common between clang modules. Make sure we test
against accidently reexporting Swift SPIs through these exported
imports.

Author: xymus

test/SPI/accidental-reexport.swift

@@ -0,0 +1,51 @@
+/// Guard that we don't reexport the SPIs accidentally through the common
+/// reexported imports between clang modules.
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -emit-module %t/Overlayed.swift -I %t -o %t
+// RUN: %target-swift-frontend -typecheck -verify %t/Client.swift -I %t
+// RUN: %target-swift-frontend -typecheck -verify %t/LowerClient.swift -I %t
+
+//--- module.modulemap
+module Overlayed {
+    header "Overlayed.h"
+}
+
+module Middle {
+    header "Middle.h"
+    export *
+}
+
+module LowerMiddle {
+    header "LowerMiddle.h"
+    export *
+}
+
+//--- Overlayed.h
+//--- Overlayed.swift
+@_exported import Overlayed
+
+public func funcPublic() {}
+
+@_spi(X)
+public func funcSPI() {}
+
+//--- Middle.h
+#include <Overlayed.h>
+
+//--- LowerMiddle.h
+#include <Middle.h>
+
+//--- Client.swift
+@_spi(X) import Middle
+
+funcPublic()
+funcSPI() // expected-error {{cannot find 'funcSPI' in scope}}
+
+//--- LowerClient.swift
+@_spi(X) import LowerMiddle
+
+funcPublic()
+funcSPI() // expected-error {{cannot find 'funcSPI' in scope}}