[HLSL] Support packoffset attribute in AST (#89836)

Add HLSLPackOffsetAttr to save packoffset in AST.

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.

For #57914.