Add a test that checks that ClangImporter chooses only exported modules (#34392)

Author: scentini

test/Interop/C/modules/print-qualified-clang-types/Inputs/HelperModule.swift

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+import ForeignA
+@_exported import ForeignB

test/Interop/C/modules/print-qualified-clang-types/Inputs/MainModule.swift

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+import HelperModule
+
+public func funcTakingForeignStruct(_ param: ForeignStruct) {}

test/Interop/C/modules/print-qualified-clang-types/Inputs/foreign-a.h

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+#include "textual-header.h"

test/Interop/C/modules/print-qualified-clang-types/Inputs/foreign-b.h

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+#include "textual-header.h"

test/Interop/C/modules/print-qualified-clang-types/Inputs/module.modulemap

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+module ForeignA {
+  // Nest the header in a sub-module to make sure these are handled correctly.
+  module Sub {
+    header "foreign-a.h"
+  }
+}
+
+module ForeignB {
+  // Nest the header in a sub-module to make sure these are handled correctly.
+  module Sub {
+    header "foreign-b.h"
+  }
+}

test/Interop/C/modules/print-qualified-clang-types/Inputs/textual-header.h

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+typedef struct {} ForeignStruct;

test/Interop/C/modules/print-qualified-clang-types/print-qualified-clang-types.swift

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+// Check that when qualifying Clang types with a module name, we choose a
+// visible module. Clang types need special treatment because multiple Clang
+// modules can contain the same type declarations from a textually included
+// header, but not all of these modules may be visible. If we choose a module
+// that isn't visible, we produce `.swiftinterface` files that don't compile.
+//
+// To test this, the test sets up the following structure:
+//
+// MainModule (Swift module)
+//   import HelperModule (Swift module)
+//     import ForeignA (Clang module)
+//       #include "textual-header.h"
+//     @_exported import ForeignB (Clang module)
+//       #include "textual-header.h"
+//
+// `ForeignA` and `ForeignB` both include the same textual header, which
+// defines the struct `ForeignStruct`.
+//
+// Because `ForeignB` is re-exported by `HelperModule`, it is visible from
+// `MainModule`, but `ForeignA` is not. This means that when `ForeignStruct` is
+// used in `MainModule`, its qualified name should be printed as
+// `ForeignB.ForeignStruct`, not `ForeignA.ForeignStruct`.
+//
+// In addition to checking for the presence of the expected string in the
+// `.swiftinterface` file, we also verify that it compiles without error.
+//
+// This is a regression test for https://bugs.swift.org/browse/SR-13032.
+
+// RUN: %empty-directory(%t)
+// RUN: mkdir %t/helper_module %t/main_module
+// RUN: %target-swift-frontend -enable-library-evolution -swift-version 5 -emit-module -o %t/helper_module/HelperModule.swiftmodule %S/Inputs/HelperModule.swift -I %S/Inputs
+// RUN: %target-swift-frontend -enable-library-evolution -swift-version 5 -emit-module -o %t/main_module/MainModule.swiftmodule -emit-module-interface-path %t/main_module/MainModule.swiftinterface -I %t/helper_module %S/Inputs/MainModule.swift -I %S/Inputs
+// RUN: %FileCheck --input-file=%t/main_module/MainModule.swiftinterface %s
+// RUN: %target-swift-frontend -typecheck -swift-version 5 %t/main_module/MainModule.swiftinterface -I %t/helper_module -I %S/Inputs
+
+// CHECK: public func funcTakingForeignStruct(_ param: ForeignB.ForeignStruct)