[ARM] Parse Tag_also_compatible_with attribute

The ARM Attribute Parser used to parse the value of also_compatible_with
as it is, disregarding the way it is encoded.

This patch does a context aware parsing of the also_compatible_with
attribute. Additionally, some error handling is also done for incorrect
cases.

Reviewed By: pratlucas

Differential Revision: https://reviews.llvm.org/D130913