[CMake] Support passing arguments to build tool (bootstrap).

For bootstrap builds (CLANG_ENABLE_BOOTSTRAP=ON) allow
arguments to be passed to the native tool used in CMake
for the stage2 step.

Can be used to pass extra arguments for enhanced versions
of build tools, e.g. distributed build options.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D131665

clang/CMakeLists.txt

@@ -787,6 +787,14 @@ if (CLANG_ENABLE_BOOTSTRAP)
     endif()
   endforeach()
 
+  # Build arguments for native tool used in CMake.
+  set(build_configuration "$<CONFIG>")
+  set(build_tool_args "${LLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS}")
+  if(NOT build_tool_args STREQUAL "")
+    string(PREPEND build_tool_args "-- ")
+    separate_arguments(build_tool_args UNIX_COMMAND "${build_tool_args}")
+  endif()
+
   ExternalProject_Add(${NEXT_CLANG_STAGE}
     DEPENDS clang-bootstrap-deps
     PREFIX ${NEXT_CLANG_STAGE}
@@ -809,6 +817,9 @@ if (CLANG_ENABLE_BOOTSTRAP)
                 ${${CLANG_STAGE}_RANLIB}
                 ${${CLANG_STAGE}_OBJCOPY}
                 ${${CLANG_STAGE}_STRIP}
+    BUILD_COMMAND ${CMAKE_COMMAND} --build ${BINARY_DIR}
+                                   --config ${build_configuration}
+                                   ${build_tool_args}
     INSTALL_COMMAND ""
     STEP_TARGETS configure build
     USES_TERMINAL_CONFIGURE 1