[AVR][clang] Fix wrong calling convention in functions return struct type

According to AVR ABI (https://gcc.gnu.org/wiki/avr-gcc), returned struct value
within size 1-8 bytes should be returned directly (via register r18-r25), while
larger ones should be returned via an implicit struct pointer argument.

Reviewed By: dylanmckay

Differential Revision: https://reviews.llvm.org/D99237