Fix: invalid alignment of enumeration fields

Fix: invalid alignment of enumeration fields

Issue
---

According to the CTF specification, the alignment of an enumeration is
that of its container integer field type. However, ctf-ir does not
forward the alignment of an enumeration field type's alignment in
bt_field_type_get_alignment().

This causes babeltrace to fail to read traces produced by lttng-ust
following a fix that causes it to generate extended event headers. The
problem is observed on ARM platforms since lttng-ust will produce
a layout that does not result in unaligned memory accesses.

Solution
---

The alignment of the enumeration field type's container is sampled
when the enumeration field type is frozen.

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