Add test for canImport submodule in mixed framework

Author: ApolloZhu

test/ClangImporter/MixedSource/Inputs/WithSubmodule.framework/Empty.swift

@@ -0,0 +1 @@
+int fromSubmodule;

test/ClangImporter/MixedSource/Inputs/WithSubmodule.framework/Headers/Submodule.h

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+framework module WithSubmodule {
+  explicit module Submodule {
+    header "Submodule.h"
+    export *
+  }
+}

test/ClangImporter/MixedSource/Inputs/WithSubmodule.framework/Modules/WithSubmodule.swiftmodule/.keep

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+// RUN: %empty-directory(%t)
+// RUN: cp -r %S/Inputs/WithSubmodule.framework %t
+// RUN: %target-swift-frontend -emit-module -o %t/WithSubmodule.framework/Modules/WithSubmodule.swiftmodule/%target-swiftmodule-name %t/WithSubmodule.framework/Empty.swift -import-underlying-module -F %t -module-name WithSubmodule
+
+// RUN: %target-typecheck-verify-swift -F %t
+
+// Testing 'canImport()' non-existing submodule in a top module loadable by other loaders.
+
+#if !canImport(WithSubmodule.Submodule)
+#error("Should can import WithSubmodule.Submodule")
+#endif
+
+// Should fail if checked for a non-existing submodule.
+#if canImport(WithSubmodule.ButNotMe)
+import WithSubmodule.Submodule
+#endif
+
+func testNotImported() {
+    fromSubmodule = 5
+    // expected-error@-1 {{cannot find 'fromSubmodule' in scope}}
+}