compiler: clean up unresolved placeholders for pointer types

compiler: clean up unresolved placeholders for pointer types

    Add a new helper routine Type::finish_pointer_types that walks through
    the pointer type cache and looks for placeholder types that may have
    been created at some point before conversion of named types, and
    invokes Type::finish_backend() on said placeholders.  This is needed
    to handle cases where the compiler manufactures a pointer type as part
    of lowering, then a placeholder is created for it due to a call to
    Type::backend_type_size(), but there is no explicit reference to the
    type in user code.

    Reviewed-on: https://go-review.googlesource.com/51131

From-SVN: r250548