[DebugInfo] generate btf_tag annotations for DISubprogram types

Generate btf_tag annotations for DISubprogram types.
A field "annotations" is introduced to DISubprogram, and
annotations are represented as an DINodeArray, similar to
DIComposite elements. The following example illustrates how
annotations are encoded in IR:
    distinct !DISubprogram(..., annotations: !10)
    !10 = !{!11, !12}
    !11 = !{!"btf_tag", !"a"}
    !12 = !{!"btf_tag", !"b"}

Differential Revision: https://reviews.llvm.org/D106618