[llvm] Add DISubrangeType (PR #126772)

[Tom Tromey via llvm-commits]

tromey wrote:

Oops, I forgot the "how they're used" part.  They are just integers:
```
   Y : Small := -5;
   Y1 : Small := -7;
```
Also worth pointing out that in Ada you can have a subrange of an enumeration type (and I think fixed point types -- but I haven't submitted the fixed point work yet).  Also, the bounds can be dynamic.

https://github.com/llvm/llvm-project/pull/126772