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

[Matt Davis via llvm-commits]

enferex wrote:

@DavidSpickett thanks for taking a look and for the suggestions.  

> Is there a reference for the structure of this debug_cu_index? 

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).


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