Turn output of -vtemplates into compiler-messages and show template instances

[Per Nordlöw]

On Thursday, 3 September 2020 at 13:20:32 UTC, jmh530 wrote:
> People also might find it useful to filter for just one 
> template (and show all the instances).

I proposed that feature as, say,

     `vtemplates=list-instances:Template1,Template2,...`

. Just complain a bit and I'll get momentum to add that too ;)