[clang] [clang] Fix behavior of `-ibuiltininc` when passed alongside `-nostdinc` (PR #165814)

[David Albert via cfe-commits]

davidbalbert wrote:

@smallp-o-p I'm curious: why did you close this PR? Was there something wrong with it? I just ran into this issue, and to my uneducated eye, your change seems like the correct behavior.

https://github.com/llvm/llvm-project/pull/165814