[dmd-internals] Add deprecation() to print deprecation messages

[Leandro Lucarella]

I wanted to point you to this pull request:
https://github.com/D-Programming-Language/dmd/pull/908

I know is not really important and shouldn't be a priority, but should
be fairly harmless to apply (passes the test suite at least) and it will
get outdated pretty fast if you don't apply it soon. Is just a cleanup
patch, it doesn't affect any functionality but it affects a lot of files
because error reporting does, and this is why it will become outdated
pretty soon.

-- 
Leandro Lucarella (AKA luca)                     http://llucax.com.ar/
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