Adjust the SAML control flow to allow LDAP identities to be added to an existing SAML user.

## What does this MR do?

It correctly lets an existing SAML user to add their LDAP identity automatically at login.

## Why was this MR needed?

A customer had issues with the `auto_link_ldap_user` feature. The flow was not working if there was an account with a SAML identity, but no LDAP identity. GitLab would pick up the correct LDAP person, but due to the order of the flow, that LDAP person was never associated with the user.

## What are the relevant issue numbers?

Fixes #17346 

/cc @dblessing @balameb @stanhu 

See merge request !4498