[Modules] [HeaderSearch] Don't reenter headers if it is pragma once (#76119)

Close https://github.com/llvm/llvm-project/issues/73023

The direct issue of https://github.com/llvm/llvm-project/issues/73023 is
that we entered a header which is marked as pragma once since the
compiler think it is OK if there is controlling macro.

It doesn't make sense. I feel like it should be sufficient to skip it
after we see the '#pragma once'.

From the context, it looks like the workaround is primarily for
ObjectiveC. So we might need reviewers from OC.