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

This reapplies
https://github.com/llvm/llvm-project/commit/c5509fedc5757fffece385d9d068e36b26793ade
"[HLSL] Support packoffset attribute in AST
(https://github.com/llvm/llvm-project/pull/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 an integer offset parameter
instead of string. This approach avoids parsing the string if the offset
is saved as a string in HLSLPackOffsetAttr.

For #57914