[clang] [clang] Enforce 1-based indexing for command line source locations (PR #139457)

[via cfe-commits]

cor3ntin wrote:

Thanks for working on that.
I think it might better to do that check where `ParsedSourceLocation::FromString` is called, so that we can have a proper
front-end diagnostics for it (search for `err_fe_invalid_code_complete_file` and `OPT_code_completion_at`)

https://github.com/llvm/llvm-project/pull/139457