[ParseableInterface] Don't serialize deps for explicit builds

If the frontend is invoked with
-build-module-from-parseable-interface, we might be trying to persist
and distribute the swiftmodule that gets built. In that case, any
dependencies we list might not be relevant.

This probably isn't really the final answer here; what we want is some
way to say /which/ dependencies are relevant, and how they're related
to how the swiftmodule that gets used. Most likely the right answer
here is to limit this to dependencies within the SDK or something.

test/ParseableInterface/SmokeTest.swiftinterface

@@ -14,6 +14,7 @@
 // RUN: %target-swift-ide-test -print-module -module-to-print SmokeTest -I %t -source-filename x -print-interface > %t/SmokeTest.txt
 // RUN: %FileCheck %s < %t/SmokeTest.txt
 // RUN: %FileCheck -check-prefix NEGATIVE %s < %t/SmokeTest.txt
+// RUN: llvm-bcanalyzer -dump %t/SmokeTest.swiftmodule | not grep FILE_DEPENDENCY
 
 // CHECK-LABEL: public class TestClass
 public class TestClass {