Remove unnecessary use of ns.follow in flattening [blocks: #3652]

[tautschnig]

Only structs, unions, enums can be hidden behind tags. And keeping symbol or tag
types in the mapping is just fine, there is no need to store the expanded
version.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: ee929f3).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96866386

[smowton]

@chrisr-diffblue can you confirm if the Ada frontend uses ID_symbol_type for primitive / array types any more?

[smowton]

Fine with me if Chris R confirms this won't break Ada

[chrisr-diffblue]

I believe we no longer use symbol types for Ada (if there are still any places we do, we'll fix them up the next time we do a submodule bump) so I'm happy for this to go in.