[flang-commits] [flang] [flang] Change `uniqueCGIdent` separator from `.` to `_` (PR #71338)

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jeanPerier wrote:

> @jeanPerier that's a good point we'd end up circling back if we encounter `$` is not valid in a given target. What about an upper case separator, or a series of upper case chars?
> I'm suggesting `X` because I think it's not in use.

I think it is a good idea. Adding @vdonaldson here to double check that there will not be collisions with this strategy.

https://github.com/llvm/llvm-project/pull/71338