[AST] Do not copy SearchPathOptions in updateNonUserModule
`updateNonUserModule` was accidentally copying `SearchPathOptions`. Take
a reference to it instead. Also, since `addFile` is actually called many
times (once for every submodule, of which there are many), change
`isNonUserModule` to a request so that it's only calculated when needed.
Resolves rdar://107155587.
