Add 'remark' diagnostic type in clang

[Arthur O'Dwyer]

On Wed, Feb 26, 2014 at 1:31 PM, Alp Toker <alp at nuanti.com> wrote:
>
> 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).

I would rephrase that like this:
Users will DEFINITELY want to upgrade a specific remark to an error,
since that's basically your intended use case: "please stop my build
if vectorizer remark <foo> is issued."  However, since Clang currently
has zero infrastructure in this area, you shouldn't try to roll that
infrastructure work into this particular patch.

"Upgrade all remarks to errors" is a special case of "upgrade remarks
<foo,bar,baz> to errors", and presumably would one day be available
via something like `-Werror=remarks` or `-Werror=everything` (and the
inverse via `-Wwarning=...` and `-Wremark=...`). Again, that's
infrastructure that doesn't exist at the moment AFAIK.

–Arthur