Introduce some infrastructure for adding C attributes with [[]] syntax.

This patch adds support to the attribute tablegen for specifying a [[]] attribute is allowed in C mode. This patch also adds the annotate attribute to the list of double square bracket attributes we support in C mode.

Eventually, I anticipate that this logic will be reversed (you have to opt out of allowing an attribute in C rather than opting in), but I want to see how the design plays out as more attributes are considered.

llvm-svn: 321763