Revert "[Modules] [HeaderSearch] Don't reenter headers if it is pragm… (#79396)

…a once  (#76119)"

This reverts commit f0c38703 because it
causes an lldb test to fail on a missing import on Mac.

https://logs.chromium.org/logs/fuchsia/buildbucket/cr-buildbucket/8758053465398947297/+/u/lldb/test/stdout