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

[David Blaikie via llvm-commits]

dwblaikie wrote:

could you check a few callers of this function to see if they'd generally be on-board with leading `+` support? (like, does this make any use cases more permissive than they intend to be/supporting things they don't intend to support)

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