SILGen: Limit references to NSError in inlinable code optimization

Revisit the optimization that provides a fast path for instances of
`NSError` when erasing the `Error` type in `emitExistentialErasure`. It
generated references to `NSError` when the `Foundation` module was
loaded, no matter how it was imported. This lead to deserialization
failures at reading the swiftmodule when that reference was added to
inlinable code while `Foundation` was not a public dependency.

Fix this crash by limiting the optimization to all non-inlinable code
and only inlinable code from a module with a public dependency on
`Foundation`. This is the similar check we apply to user written
inlinable code, however here we use the module-wide dependency instead
of per file imports.

rdar://142438679

lib/SILGen/SILGenConvert.cpp

@@ -656,9 +656,10 @@ ManagedValue SILGenFunction::emitExistentialErasure(
   // If we're erasing to the 'Error' type, we might be able to get an NSError
   // representation more efficiently.
   auto &ctx = getASTContext();
+  auto *nsErrorDecl = ctx.getNSErrorDecl();
   if (ctx.LangOpts.EnableObjCInterop && conformances.size() == 1 &&
       conformances[0].getRequirement() == ctx.getErrorDecl() &&
-      ctx.getNSErrorDecl()) {
+      nsErrorDecl && referenceAllowed(nsErrorDecl)) {
     // If the concrete type is NSError or a subclass thereof, just erase it
     // directly.
     auto nsErrorType = ctx.getNSErrorType()->getCanonicalType();