[AVR] Add AVRMCExpr

Summary: This adds the AVRMCExpr headers and implementation.

Reviewers: arsenm, ruiu, grosbach, kparzysz

Subscribers: wdng, beanz, mgorny, kparzysz, jtbandes, llvm-commits

Differential Revision: https://reviews.llvm.org/D20503

llvm-svn: 282397