Add 'remark' diagnostic type in clang

[Chris Lattner]

On Feb 26, 2014, at 1:31 PM, Alp Toker <alp at nuanti.com> wrote:
>>> It’s a bit odd, but since these are diagnostics, why not use the existing -W flags?  You should be able to -Werror one of these, control them with #pragma clang diagnostics, etc.  It doesn’t seem like we need more complexity in this space.
>> 
>> Good point. I will prepare the above patches such that they reuse the existing infrastructure. If we really see a need for further adjustments, we can do this incrementally.
> 
> Yes, this is sounding good. Keep in mind that it may be preferable not to permit upgrading remarks to errors (another potential use case of this being informational diagnostics about system headers that aren't user-actionable).

Warnings already default to being suppressed in system headers, remarks should default to the same behavior.

-Chris