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

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vdonaldson wrote:

Given the discussion to this point, I'm inclined to favor `X`. That should avoid collisions.

The derived type info use case may be the most significant, the most common, and the most visible. There are likely a few other places that use `.` in compiler-generated symbol names beyond those already mentioned. There are two instances of `.list` suffixes in `lib/Lower/IO.cpp`.  The first one is [here](https://github.com/llvm/llvm-project/blob/58679ea576f9e48e20e43dcd99fd75c98fb7e6ba/flang/lib/Lower/IO.cpp#L324).

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