DebugInfo: Add comments/assert description to r209674 based on Eric Christopher's post-commit review feedback.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210228 91177308-0d34-0410-b5e6-96231b3b80d8
Use AArch64 instead of now removed ARM64 in test configs

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210229 91177308-0d34-0410-b5e6-96231b3b80d8