[clang] [clang][lex] Fix non-portability diagnostics with absolute path (PR #74782)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

@jrmwng PR #76985 (landed in 853b133) seems to have resolved this issue for me. Can you confirm that on your end?

https://github.com/llvm/llvm-project/pull/74782