[ARM] Rewrite ARMAttributeParser

* Delete boilerplate
* Change functions to return `Error`
* Test parsing errors
* Update callers of ARMAttributeParser::parse() to check the `Error` return value.

Since this patch touches nearly everything in the file, I apply
http://llvm.org/docs/Proposals/VariableNames.html and change variable
names to lower case.

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D75015