[clang] [clang] Add test for CWG2811 "Clarify "use" of main" (PR #96168)

[via cfe-commits]

https://github.com/cor3ntin approved this pull request.

LGTM if @AaronBallman is happy

("expressions that... are an extensions" still sounds weird to me, cf previous comment)

https://github.com/llvm/llvm-project/pull/96168