[DebugInfo] generate btf_tag annotations for DIComposite types

Clang patch D106614 added attribute btf_tag support. This patch
generates btf_tag annotations for DIComposite types.
A field "annotations" is introduced to DIComposite, and the
annotations are represented as an DINodeArray, similar to
DIComposite elements. The following example illustrates
how annotations are encoded in IR:
  distinct !DICompositeType(..., annotations: !10)
  !10 = !{!11, !12}
  !11 = !{!"btf_tag", !"a"}
  !12 = !{!"btf_tag", !"b"}
Each btf_tag annotation is represented as a 2D array of
meta strings. Each record may have more than one
btf_tag annotations, as in the above example.

Differential Revision: https://reviews.llvm.org/D106615