De-duplicating Error messages

[Walter Bright]

This is one of the priority focuses with improving the user experience with the 
compiler.

I tried a couple general approaches:

https://github.com/dlang/dmd/pull/15303
https://github.com/dlang/dmd/pull/15306

but they both failed for overdoing things, for various reasons. Fixing those 
reasons just seemed to be more trouble than they are worth.

I switched to the rifle shot approach,

https://github.com/dlang/dmd/pull/15312

which is about picking out specific troublesome messages and deduplicating just 
them.

If you have specific examples of duplicated error messages, please let me know.