Dedupe types when printing error messages

[ProkopRandacek]

Hi! :D

Before:

file.zig:10:9: error: expected type '*type_dedupe.SomeVeryLongName', found 'type_dedupe.SomeVeryLongName'
    foo(a);
        ^

After:

file.zig:10:9: error: expected type '*<T>' but found '<T>'
    foo(a);
        ^
file.zig:10:9: note: <T> = type_dedupe.SomeVeryLongName

Previously discussed on zulip: #compiler > Deduped type comparisons in error messages

The placeholder formatting is ready to be bikeshedded. I would like to add more color but I'm not sure how to do that in a way that would respect no-color options from the cli and such.

I'm also not sure if I got all the places where sema diffs types.