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

[Per Nordlöw]

D just got a better formatting of existing template usage sorted 
by descending unique instantiation count via the flag 
`-vtemplates`. Each template that has a least one instantiation 
is now printed using standard compiler diagnostics formatting for 
easy navigation to the point of its declaration.

Additionally, a new feature is to instead of

     `-vtemplates`

use

     `-vtemplates=list-instances`

. Then the location(s) for each instance of each instantiated 
template is printed along with information telling whether the 
instantiation was implicit or explicit.

A good example of the behavior can be found in the test file [2]

[1] https://github.com/dlang/dmd/pull/11463
[2] 
https://github.com/nordlow/dmd/blob/6b69ff7f0676314953344b5c920d960c023b3134/test/compilable/vtemplates_list.d