[clang] -fstack-usage: fix filename for functions in an included file (PR #69896)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> Thanks for the MR, @MaskRay! The patch LGTM. Can you confirm that the failing checks are not related to this patch?

Thanks for review. I confirm it is an unrelated `clang-format` issue (`grep -rnI '[[:blank:]]$' clang/lib clang/include clang/docs`) caused by another patch and fixed now.

https://github.com/llvm/llvm-project/pull/69896