[clang] [HLSL] reenable add packoffset in AST (PR #91474)

[Justin Bogner via cfe-commits]

bogner wrote:

When reapplying a change that was reverted, please include enough information in the description so that it can be understood without having to dig up the old change. Something like:

> [HLSL] reenable add packoffset in AST
>
> This reapplies c5509fedc575 "[HLSL] Support packoffset attribute in AST (#89836)" with a fix for the test failure caused by missing -fnative-half-type.
> 
> Since we have to parse the attribute manually in ParseHLSLAnnotations, we could create the ParsedAttribute with a integer offset parameter instead of string. This approach avoids parsing the string if the offset is saved as a string in HLSLPackOffsetAttr.

https://github.com/llvm/llvm-project/pull/91474