compiler: use the underlying type to build placeholder type for alias
When asking for a placeholder type of an alias type, build a
placeholder for the underlying type, instead of treating the
alias as a named type and calling get_backend. The latter may
fail as we may not be ready to build a complete backend type. We
have already used a unified backend type for alias type and its
underlying type. Do the same for placeholders as well.
Reviewed-on: https://go-review.googlesource.com/138635
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@264773 138bc75d-0d04-0410-961f-82ee72b054a4