Record parseable interface imports in the debug info.

When a Swift module built with debug info imports a library without
debug info from a textual interface, the textual interface is
necessary to reconstruct types defined in the library's interface.  By
recording the Swift interface files in DWARF dsymutil can collect them
and LLDB can find them.

rdar://problem/49751363

lib/Serialization/SerializedModuleLoader.cpp

@@ -394,6 +394,7 @@ FileUnit *SerializedModuleLoaderBase::loadAST(
     // We've loaded the file. Now try to bring it into the AST.
     auto fileUnit = new (Ctx) SerializedASTFile(M, *loadedModuleFile,
                                                 extendedInfo.isSIB());
+    fileUnit->setParseableInterface(extendedInfo.getParseableInterface());
     M.addFile(*fileUnit);
     if (extendedInfo.isTestable())
       M.setTestingEnabled();