Print compiler version in diagnostics when certain code is compiled

[jcouv]

It would be useful to troubleshooting purpose to have the equivalent of phpversion() for C# and VB.
It could be a special pragma (#VERSION), a well-known method without implementation (System.Runtime.CompilerServices.RuntimeHelpers.Version()?) or maybe a special doc tag.

The output should not only include the version of the compiler, but also the language version that is used to compile the current code (this would be useful in asp.net scenario).

FYI @gafter @jaredpar

[jcouv]

Fixed by #18045

With the C# compiler, use #error version and you will get a diagnostic including the compiler and language version this code is getting compiled with.