[CodeCompletion] Avoid suggesting duplicated module names

in global completion result. Overly modules have the same name as the
shadowed modules. We should not list both names because they are
identical.

Maintain a set of seen module names to avoid suggesting duplicated
names.

rdar://problem/63370253

test/IDE/Inputs/mock-sdk/OverlayTest.framework/Modules/module.modulemap

@@ -0,0 +1,7 @@
+framework module OverlayTest {
+  umbrella header "OverlayTest.h"
+
+  export *
+  module * { export * }
+}
+