[ParseableInterface] Add test for #if canImport

This works correclty as-is, but it should be tested.

rdar://48733713

Author: Harlan Haskins

test/ParseableInterface/can-import.swift

@@ -0,0 +1,14 @@
+// RUN: %empty-directory(%t)
+// RUN: echo 'public func externalFunc() {}' | %target-swift-frontend -typecheck -emit-parseable-module-interface-path %t/Library.swiftinterface -
+// RUN: %target-swift-frontend -typecheck %s -I %t
+
+#if canImport(Library)
+import Library
+externalFunc()
+#else
+#error("unable to import Library from its parseable interface")
+#endif
+
+#if canImport(LibraryThatDoesNotExist)
+#error("should not return true for library that does not exist")
+#endif