CORRADE_DEPRECATED_*() macros don't really work on MSVC2015

[mosra]

Reported by @AVK330.

• Using __declspec instead of [[]] might make it working in some cases but might require a different placement in other cases (in that case the macro would need to be completely no-op on 2015). nope
• Verify that the other macros work properly (it seems that the test doesn't trigger deprecation warning for structs) well, partly
• Verify the same on MSVC2017 and GCC (I think there are also some bad cases) -- yes.

[mosra]

Looked at this, and ... the support is spotty at best, deprecation warnings emitted only in some cases but not all (and can't really do anything about that, compiler limitations). The docs are updated to mention these in 229693f, Also online.