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

[Volodymyr Sapsai via cfe-commits]

https://github.com/vsapsai approved this pull request.

Overall, the change works fine and it is pretty small. Have a few minor suggestion for the test but nothing blocking.

https://github.com/llvm/llvm-project/pull/76119