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
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@250683 138bc75d-0d04-0410-961f-82ee72b054a4