[CMake] Fix cross-compiling with LLVM as CMake subproject

[Flakebi]

Summary

When embedding LLVM as a CMake subproject, using cross-compiling does
not work at the moment. This also affects -DLLVM_OPTIMIZED_TABLEGEN=1,
which uses the same CMake infrastructure.

This patch replaces global CMake variables with the current version,
which allows cross-compilation to work in a subproject.

CMAKE_BINARY_DIR -> CMAKE_CURRENT_BINARY_DIR
CMAKE_SOURCE_DIR -> CMAKE_CURRENT_SOURCE_DIR
CMAKE_PROJECT_NAME -> PROJECT_NAME

[nhaehnle]

I'm no CMake expert, but this seems pretty plausible to me. LGTM -- though you'll definitely want to keep an eye out for bot failures.

llvm/cmake/modules/CrossCompile.cmake

 include(AddLLVM)
 include(LLVMExternalProjectUtils)
 
 
 function(llvm_create_cross_target project_name target_name toolchain buildtype)
 
   if(NOT DEFINED ${project_name}_${target_name}_BUILD)
     set(${project_name}_${target_name}_BUILD
-      "${CMAKE_BINARY_DIR}/${target_name}")
+      "${CMAKE_CURRENT_BINARY_DIR}/${target_name}")
     set(${project_name}_${target_name}_BUILD
       ${${project_name}_${target_name}_BUILD} PARENT_SCOPE)
     message(STATUS "Setting native build dir to " ${${project_name}_${target_name}_BUILD})
   endif(NOT DEFINED ${project_name}_${target_name}_BUILD)
 
   if (EXISTS ${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake)
     set(CROSS_TOOLCHAIN_FLAGS_INIT
       -DCMAKE_TOOLCHAIN_FILE=\"${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake\")
foreach(project ${LLVM_EXTERNAL_PROJECTS})
     canonicalize_tool_name(${project} name)
     list(APPEND external_project_source_dirs
          "-DLLVM_EXTERNAL_${name}_SOURCE_DIR=${LLVM_EXTERNAL_${name}_SOURCE_DIR}")
   endforeach()
 
   add_custom_command(OUTPUT ${${project_name}_${target_name}_BUILD}/CMakeCache.txt
     COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}"
         -DCMAKE_MAKE_PROGRAM="${CMAKE_MAKE_PROGRAM}"
-        ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
+        ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_CURRENT_SOURCE_DIR}
         ${CROSS_TOOLCHAIN_FLAGS_${project_name}_${target_name}}
         -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
         -DLLVM_TARGETS_TO_BUILD="${targets_to_build_arg}"
         -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD="${experimental_targets_to_build_arg}"
         -DLLVM_DEFAULT_TARGET_TRIPLE="${TARGET_TRIPLE}"
         -DLLVM_TARGET_ARCH="${LLVM_TARGET_ARCH}"
         -DLLVM_ENABLE_PROJECTS="${llvm_enable_projects_arg}"
         -DLLVM_EXTERNAL_PROJECTS="${llvm_external_projects_arg}"

 # LLVM_OPTIMIZED_TABLEGEN. Always builds in Release.
 # - target: The target to build natively
 # - output_path_var: A variable name which receives the path to the built target
 # - DEPENDS: Any additional dependencies for the target
 function(build_native_tool target output_path_var)
   cmake_parse_arguments(ARG "" "" "DEPENDS" ${ARGN})
 
   if(CMAKE_CONFIGURATION_TYPES)
-    set(output_path "${${CMAKE_PROJECT_NAME}_NATIVE_BUILD}/Release/bin/${target}")
+    set(output_path "${${PROJECT_NAME}_NATIVE_BUILD}/Release/bin/${target}")
   else()
-    set(output_path "${${CMAKE_PROJECT_NAME}_NATIVE_BUILD}/bin/${target}")
+    set(output_path "${${PROJECT_NAME}_NATIVE_BUILD}/bin/${target}")
   endif()
 
-  llvm_ExternalProject_BuildCmd(build_cmd ${target} ${${CMAKE_PROJECT_NAME}_NATIVE_BUILD}
+  llvm_ExternalProject_BuildCmd(build_cmd ${target} ${${PROJECT_NAME}_NATIVE_BUILD}
                                 CONFIGURATION Release)
   add_custom_command(OUTPUT "${output_path}"
                      COMMAND ${build_cmd}
-                     DEPENDS CONFIGURE_${CMAKE_PROJECT_NAME}_NATIVE ${ARG_DEPENDS}
-                     WORKING_DIRECTORY "${${CMAKE_PROJECT_NAME}_NATIVE_BUILD}"
+                     DEPENDS CONFIGURE_${PROJECT_NAME}_NATIVE ${ARG_DEPENDS}
+                     WORKING_DIRECTORY "${${PROJECT_NAME}_NATIVE_BUILD}"
                      COMMENT "Building native ${target}..."
                      USES_TERMINAL)
   set(${output_path_var} "${output_path}" PARENT_SCOPE)
 endfunction()