Issue Description:

Reproducer: consider any SIL file with import declarations, excluding the hardcoded modules Builtin, Swift, SwiftShims.

// foo.sil
sil_stage raw
import _Differentiation

Running sil-opt by itself preserves the import declaration:

$ sil-opt test.sil
sil_stage raw

import Builtin
import Swift
import SwiftShims

import _Differentiation

But generating a .sib file and deserializing it does not preserve the import declaration:

$ sil-opt -emit-sib test.sil -o test.sib
$ sil-opt -module-name main
sil_stage raw

import Builtin
import Swift
import SwiftShims

This is an issue for round-trip SIL deserialization tests that use import declarations, since the import declarations are not preserved.