[NFC] Fix a mem-sanitizer found issue in AutoType

We only need the list of constriant template arguments when we have a
valid constraint.  We give up on merging the auto-type constraints if
the template arguments don't match, but neglected to clear the
collection of template arguments.  The result was we had an AutoType
where we initialized the number of template arguments, but never
initialized the template arguments themselves.

This patch adds an assert to catch this in the future, plus ensures we
clear out the vector so we don't try to create the AutoType incorrectly.