[AVR] Add support for the full set of inline asm constraints

Summary:
Previously the method would simply return false, causing every single
inline assembly constraint to trigger a compile error.

This adds inline assembly constraint support for the AVR target.

This patch is derived from the code in
AVRISelLowering::getConstraintType.

More details can be found on the AVR-GCC reference wiki
http://www.nongnu.org/avr-libc/user-manual/inline_asm.html

Reviewers: jroelofs, asl

Reviewed By: asl

Subscribers: asl, ahatanak, saaadhu, cfe-commits

Differential Revision: https://reviews.llvm.org/D28344

llvm-svn: 294176