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

smallp-o-p wants to merge 2 commits intollvm:mainllvm/llvm-project:mainfrom

smallp-o-p:fix-ibuiltininc-nostdincsmallp-o-p/llvm-project:fix-ibuiltininc-nostdincCopy head branch name to clipboard