[flang-commits] [flang] [flang][build] Fixed paths discrovery for the out-of-tree build. (PR #87822)

[Slava Zakharin via flang-commits]

vzakhari wrote:

I see what happened.  This change actually fixed handling of the relative paths, but this buildbot was using a trick to workaround the issue that was fixed.

The buildbot builds LLVM tree here: `/home/tcwg-buildbot/worker/flang-aarch64-out-of-tree/build_llvm`.  So the projects' config files are present in `/home/tcwg-buildbot/worker/flang-aarch64-out-of-tree/build_llvm/lib/cmake`.  The out-of-tree Flang build is configured like this:
```
cmake ../llvm-project/flang -DFLANG_ENABLE_WERROR=ON -DCMAKE_BUILD_TYPE=Release -GNinja -DLLVM_DIR:PATH=../../build_llvm/lib/cmake/llvm -DMLIR_DIR:PATH=../../build_llvm/lib/cmake/mlir -DCLANG_DIR:PATH=../../build_llvm/lib/cmake/clang
```
And it is run from `/home/tcwg-buildbot/worker/flang-aarch64-out-of-tree/build_flang`, so the path specified by `CLANG_DIR` does not really point to where the config files are.  It points to `/home/tcwg-buildbot/worker/build_llvm/lib/cmake/clang`.

The buildbot needs to be fixed, since we can now use "real" paths relative to the Flang build directory.

https://github.com/llvm/llvm-project/pull/87822