[clang] [clang] use relative paths for builtin headers during module compilation (PR #68023)

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

Thanks for your patience, I should have looked at this sooner. The change to our downstream test looks expected; the `<module-includes>` buffer changed
```diff
-#import "/^tc/lib/clang/18/include/stdbool.h"
+#import "stdbool.h"
```

But the actual resolution of which header is used is identical.

I'm happy with this change; @jansvoboda11 were you looking at anything else or is this okay for @rmaz to land?

https://github.com/llvm/llvm-project/pull/68023