DebugInfo: Sure up subprogram variable list handling with more assertions and fewer conditionals.

Many old tests using prior schemas still had some brokenness here (both
indirect arrays and arrays with single bogus elements). Fixed those up
so they don't hit the new assertions.

Also reduced nesting in some places, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208817 91177308-0d34-0410-b5e6-96231b3b80d8