ir: Implement a field type visitor to resolve dynamic scopes

ir: Implement a field type visitor to resolve dynamic scopes

Implement a visitor looking for sequences' length field and variants'
tag names and resolve them to their proper type field in the trace.

Validate that sequence lengths are unsigned integers and variant
tags are enumerations and that both respect CTF's lexical and dynamic
scoping rules.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>