[llvm] [lit] Fix use_lld() to respect llvm_shlib_dir

[mgorny]

Summary

Fix the use_lld() to use llvm_shlib_dir similarly to how use_clang()
does it. This fixes use_lld() wrongly prepending llvm_libs_dir,
i.e. the directory with system-installed LLVM libraries before
the build directory of standalone build. As a result, the shared
libraries from an earlier version of clang end up being used instead of
the newly built version when running the test suite prior to installing.

To reproduce the problem, build and install LLVM with dylibs first,
e.g.:

cmake ../llvm -G Ninja -DCMAKE_BUILD_TYPE=MinSizeRel \
  -DCMAKE_INSTALL_PREFIX="${HOME}"/llvm-test \
  -DLLVM_BUILD_LLVM_DYLIB=ON -DLLVM_LINK_LLVM_DYLIB=ON \
  -DLLVM_INSTALL_UTILS=ON
ninja install

Then build clang against that installation and run tests:

export LD_LIBRARY_PATH=~/llvm-test/lib
export PATh=~/llvm-test/bin:"${PATH}"
cmake ../clang -G Ninja -DCMAKE_BUILD_TYPE=MinSizeRel \
  -DCMAKE_INSTALL_PREFIX="${HOME}"/llvm-test \
  -DCLANG_LINK_CLANG_DYLIB=ON -DLLVM_BUILD_TESTS=ON \
  -DLLVM_EXTERNAL_LIT="${PWD}"/bin/llvm-lit
ninja check-clang

The tests will be run with LD_LIBRARY_PATH of:

/home/${USER}/llvm-test/lib:/home/${USER}/llvm-project/build-clang/lib

As a result, installed libclang-cpp will take precedence over the one
from build dir. With the patch, the correct path is used, i.e.:

/home/${USER}/llvm-project/build-clang/lib:/home/${USER}/llvm-test/lib

[MaskRay]

Is there a CMake command to demonstrate the difference? Consider adding it to the description.

[mgorny]

In D135368#3840429, @MaskRay wrote:

Is there a CMake command to demonstrate the difference? Consider adding it to the description.

Do you mean the standalone build method that could trigger this problem, or something that actually triggers a test failure?

[MaskRay]

In D135368#3840437, @mgorny wrote:

In D135368#3840429, @MaskRay wrote:

Is there a CMake command to demonstrate the difference? Consider adding it to the description.

Do you mean the standalone build method that could trigger this problem, or something that actually triggers a test failure?

A small reproducible instruction, possibly two CMake commands, one with llvm/CMakeListss.txt and (for standalone build) the other with lld/CMakeLists.txt

[mgorny]

In D135368#3840486, @MaskRay wrote:

A small reproducible instruction, possibly two CMake commands, one with llvm/CMakeListss.txt and (for standalone build) the other with lld/CMakeLists.txt

Done. Although it's clang what's failing here.

llvm/utils/lit/lit/llvm/config.py

def use_lld(self, additional_tool_dirs=[], required=True,
         exe_dir_props = [self.config.name.lower() + '_tools_dir',
                          'lld_tools_dir', 'llvm_tools_dir']
         paths = [getattr(self.config, pp) for pp in exe_dir_props
                  if getattr(self.config, pp, None)]
         paths = additional_tool_dirs + paths
         self.with_environment('PATH', paths, append_path=True)
 
         lib_dir_props = [self.config.name.lower() + '_libs_dir',
-                         'lld_libs_dir', 'llvm_libs_dir']
+                         'lld_libs_dir', 'llvm_shlib_dir', 'llvm_libs_dir']
         lib_paths = [getattr(self.config, pp) for pp in lib_dir_props
                      if getattr(self.config, pp, None)]
 
         self.with_environment('LD_LIBRARY_PATH', lib_paths, append_path=True)
 
         # Discover the LLD executables to use.
 
         ld_lld = self.use_llvm_tool('ld.lld', required=required,