[clang] [clang][clang-scan-deps] Add LangOptions::AllowLiteralDigitSeparator to fix (#88896) (PR #158420)

[Naveen Seth Hanig via cfe-commits]

naveen-seth wrote:

> This LGTM, cheers!

Hi @tsfn, @jansvoboda11,

I also ran into the issue this fixes today (specifically https://github.com/NVIDIA/stdexec/issues/1638) and was looking for a fix.
This PR seems to have been approved, but was never merged.

@tsfn, would you still like to work on this? 
If not, would it be okay for me to recreate this PR, since the original repository has been deleted?

https://github.com/llvm/llvm-project/pull/158420