[libcxx-commits] [libcxx] [libcxx][clang-modules] Fix headers being marked as textual (PR #130723)

[Louis Dionne via libcxx-commits]

ldionne wrote:

> What is LIBCXX_GENERATED_INCLUDE_TARGET_DIR actually used for? 

It's used to ship differently-configured libc++'s on different targets. We have:

```
include/c++/v1/
              vector
              string
              etc..
include/c++/v1/aarch64-linux-whatever/
              __config_site
include/c++/v1/x86_64-linux-whatever/
              __config_site
```

The compiler then adds `-isystem <ROOT>/include/c++/v1 -isystem <ROOT>/include/c++/v1/<target>` based on the `--target` it gets passed. We don't use it on Apple platforms (yet).

Note that I am not concerned with the modulemap not working from the build directory. There might be a few challenges to deploying that (I think when @arichardson last tried he found some places in LLVM that relied on the structure of our build directory), but in principle we should never use anything from our build directory, only from the installed result.

https://github.com/llvm/llvm-project/pull/130723