[BPF] add unit tests for preserve_{array,union,struct}_access_index intrinsics

[BPF] add unit tests for preserve_{array,union,struct}_access_index intrinsics

This is a followup patch for https://reviews.llvm.org/D61810/new/,
which adds new intrinsics preserve_{array,union,struct}_access_index.

Currently, only BPF backend utilizes preserve_{array,union,struct}_access_index
intrinsics, so all tests are compiled with BPF target.

https://reviews.llvm.org/D61524 already added some tests for these
intrinsics, but some of them pretty complex.
This patch added a few unit test cases focusing on individual intrinsic
functions.

Also made a few clarification on language reference for these intrinsics.

Differential Revision: https://reviews.llvm.org/D64606

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366038 91177308-0d34-0410-b5e6-96231b3b80d8