[DebugInfo] Add verifier for DICompositeType vector

Summary:
This patch adds verification logic for DICompositeType vectors, ensuring that they only have one element, and that element is of type subrange.

This patch complements https://reviews.llvm.org/D44048

Reviewers: aprantl

Reviewed By: aprantl

Subscribers: JDevlieghere, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D44262

llvm-svn: 327048