[clang] [Clang] Fix crash when -header-include-filtering is not specified (PR #136232)

[Bob Wilson via cfe-commits]

bob-wilson wrote:

I guess we could. Do you think it's worth it? These are only internal cc1 flags. I don't mind to change it, but it will add some minor complexity.

https://github.com/llvm/llvm-project/pull/136232