Fix T75810: Child bone frozen when both Auto IK and X-Axis mirror are

used

Caused by {rBa6a9a12e8f32}

Other relevant commits:
rBb8ca806b7798e2f8dd6effca8f0d081b3cd8c23f
rBde530a95dc7b482dc22c933b9b8b2a98c79b5663

The issue is caused by some leftover BONE_TRANSFORM_MIRROR flags on a
bone from previous runs (file in the report had the flag still on
forearm.R).

With these false leftover flags still set, `pose_grab_with_ik()` cannot
work correctly. Culprit commit above removed the early clearing of this
flag on all bones, this should be restored [this happened in
`count_set_pose_transflags()`].

This should only be done in the beginning of the transform process, so
now still clear the flags early in 'createTransPose()' [but dont restore
this in 'count_set_pose_transflags()' -- this will be called from
special_aftertrans_update again, so placing the clearance here only
complicates things (autokeyframe_pose() still needs to work as well)...]

Maniphest Tasks: T75810

Differential Revision: https://developer.blender.org/D7527

source/blender/editors/transform/transform_convert_armature.c

@@ -685,6 +685,9 @@ void createTransPose(TransInfo *t)
     if (mirror) {
       int total_mirrored = 0;
       LISTBASE_FOREACH (bPoseChannel *, pchan, &ob->pose->chanbase) {
+        /* Clear the MIRROR flag from previous runs. */
+        pchan->bone->flag &= ~BONE_TRANSFORM_MIRROR;
+
         if ((pchan->bone->flag & BONE_TRANSFORM) &&
             BKE_pose_channel_get_mirrored(ob->pose, pchan->name)) {
           total_mirrored++;