[llvm] [DebugInfo][DWARF] Bounds-check the unit-index parallel table (PR #213285)

[David Spickett via llvm-commits]

DavidSpickett wrote:

> The format of the .debug_cu_index section, including the header fields, is located in section 7.3.5.2 (pdf page 209, doc page 191) [here](https://dwarfstd.org/doc/DWARF5.pdf).

Thanks. The test now makes more sense to me.

https://github.com/llvm/llvm-project/pull/213285