[Clang][Attr] rename btf_tag to btf_decl_tag

Current btf_tag is applied to declaration only.
Per discussion in https://reviews.llvm.org/D111199,
we plan to introduce btf_type_tag attribute for types.
So rename btf_tag to btf_decl_tag to make it easily
differentiable from btf_type_tag.

Differential Revision: https://reviews.llvm.org/D111588