[compiler-rt] Use locally configured llvm-lit for standalone builds (#83178)

When building a standalone build with
`-DLLVM_CMAKE_DIR=$HOME/output/llvm-install
-DCOMPILER_RT_INCLUDE_TESTS=ON`, the current code will attempt to use
`LLVM_DEFAULT_EXTERNAL_LIT` which is set to
`$HOME/output/llvm-install/bin/llvm-lit` inside `LLVMConfig.cmake` even
though it is not actually installed. If we are adding the llvm-lit
subdirectory, we can use `get_llvm_lit_path()` immediately afterwards to
set LLVM_EXTERNAL_LIT so that subsequent calls within
`add_lit_testsuite()` use llvm-lit from the current build directory
instead of the nonexistant one.

Author: arichardson

compiler-rt/CMakeLists.txt

@@ -771,8 +771,6 @@ mark_as_advanced(COMPILER_RT_ENABLE_INTERNAL_SYMBOLIZER)
 add_subdirectory(lib)
 
 if(COMPILER_RT_INCLUDE_TESTS)
-  add_subdirectory(unittests)
-  add_subdirectory(test)
   # Don't build llvm-lit for runtimes-build, it will clean up map_config.
   if (COMPILER_RT_STANDALONE_BUILD AND NOT LLVM_RUNTIMES_BUILD)
     # If we have a valid source tree, generate llvm-lit into the bin directory.
@@ -782,11 +780,17 @@ if(COMPILER_RT_INCLUDE_TESTS)
       # Needed for lit support in standalone builds.
       include(AddLLVM)
       add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/llvm-lit ${CMAKE_CURRENT_BINARY_DIR}/llvm-lit)
+      # Ensure that the testsuite uses the local lit rather than
+      # LLVM_INSTALL_DIR/bin/llvm-lit (which probably does not exist).
+      get_llvm_lit_path(_base_dir _file_name)
+      set(LLVM_EXTERNAL_LIT "${_base_dir}/${_file_name}" CACHE STRING "Command used to spawn lit" FORCE)
     elseif(NOT EXISTS ${LLVM_EXTERNAL_LIT})
       message(WARNING "Could not find LLVM source directory and LLVM_EXTERNAL_LIT does not"
                        "point to a valid file.  You will not be able to run tests.")
     endif()
   endif()
+  add_subdirectory(unittests)
+  add_subdirectory(test)
 endif()
 
 add_subdirectory(tools)