[llvm] [Support] Recognise the '+' char for positive integers (PR #135856)

[David Blaikie via llvm-commits]

dwblaikie wrote:

You could try a local change /requiring/ the `+` to see which tests fail, then follow-up with owners in those areas (or one or two of the most prominent ones at least) to see if this optional leading `+` would be acceptable for their use case?

https://github.com/llvm/llvm-project/pull/135856