Revert "[Clang] Ignore BTFTag attr if used as a type attribute"

This reverts commit b8753438.

Per discussion in https://reviews.llvm.org/D111199, instead to make
existing btf_tag attribute as a type-or-decl attribute, we will
make existing btf_tag attribute as a decl only attribute, and
introduce btf_type_tag as a type only attribute. This will make
it easy for cases like typedef where an attribute may be applied
as either a type attribute or a decl attribute.