Use Flags object on file format flags

[nijel]

Describe the problem

The current flags interface in file formats (TranslationUnit) returns strings. The only reason for this is that this API predates the introduction of the Flags object. This makes inheritance unnecessary complex and brings in repeated Flags parsing and serializing.

Describe the solution you would like

• Change TranslationUnit.flags to return Flags objects.
• Change the overriden methods to extend super() return value to ease implementation of features like Automatic flags based on string location #13597.
• Add explicit flags serialization to string in Unit.update_source_unit.

Describe alternatives you have considered

No response

Screenshots

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Additional context

No response