[AVR] Add support for the 'interrupt' and 'naked' attributes

Summary:
This teaches clang how to parse and lower the 'interrupt' and 'naked'
attributes.

This allows interrupt signal handlers to be written.

Reviewers: aaron.ballman

Subscribers: malcolm.parsons, cfe-commits

Differential Revision: https://reviews.llvm.org/D28451

llvm-svn: 294402