Implement N3760, support for the [[deprecated]] attribute.

This motion appears to have consensus in the C++ committee and is
expected to be voted into the C++14 draft tomorrow during the formal
vote.

No extension warning in C++11 as attributes not specified in the
standard using the standard syntax in C++11 have implementation defined
meaning, and we define the meaning proposed for C++14.

There is one bug exposed as we do not warn on a deprecated template
specialization. I've left a FIXME in the test case and will dig into
that next.

Also, I will add a few more exhaustive test cases of the other features
specified for this attribute in a follow-up commit.