PR29079 shows that pretty printers can be used for an incomplete
type (stub), but only when printing it directly, not if it's
part of another struct:
```
(gdb) p s
$1 = {pp m_i = 5}
(gdb) p s2
$2 = {m_s = <incomplete type>, m_l = 20}
```
The reason is simply that in common_val_print the check for stubs
is before any pretty printer is tried.
It works if the pretty printer is tried before the stub check:
```
(gdb) p s
$1 = {pp m_i = 5}
(gdb) p s2
$2 = {m_s = {pp m_i = 10}, m_l = 20}
```
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29079
Approved-By: Tom Tromey <tom@tromey.com>
7543c960b0