[clang] NFC: Make clang resource headers an interface library (PR #88317)

[Jonas Devlieghere via cfe-commits]

JDevlieghere wrote:

Hey @etcwilde, this breaks the Xcode standalone build for lldb: https://green.lab.llvm.org/job/llvm.org/view/LLDB/job/lldb-cmake-standalone/121/ 

The failing command in the generated script phase is:

```
cmake -E copy_directory  /Users/jonas/llvm/xcode-build/Debug/bin/LLDB.framework/Versions/A/Resources/Clang/include
```

It looks like it's missing the source directly. Mind taking a look?

https://github.com/llvm/llvm-project/pull/88317