compiler: track placeholder pointer types for conversion

compiler: track placeholder pointer types for conversion

    We recently started walking through the hash table of pointer types to
    finalize them.  Unfortunately it is possible to create a new pointer
    type while finalizing an existing one (test case: test/fixedbugs/issue5291)
    and that breaks the iteration.  So, instead, keep a list of
    placeholder pointer types, and iterate through them while permitting
    the list to be extended as we go.

    Reviewed-on: https://go-review.googlesource.com/51771

From-SVN: r250683