Model type attributes as regular Attrs.

Model type attributes as regular Attrs.

Specifically, AttributedType now tracks a regular attr::Kind rather than
having its own parallel Kind enumeration, and AttributedTypeLoc now
holds an Attr* instead of holding an ad-hoc collection of Attr fields.

Differential Revision: https://reviews.llvm.org/D50526

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@339623 91177308-0d34-0410-b5e6-96231b3b80d8