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

[CarlosAlbertoEnciso]

Summary

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.

[russell.gallop]

Noting related review: https://reviews.llvm.org/D115815 which added this variable to support this for other "external projects".

[CarlosAlbertoEnciso]

To give more scope to the reviewers, the following CMake options illustrate the benefit of this patch.

For example, if I want to do a dry run on stage-2, I will pass -DLLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS="-n --verbose"

-DCLANG_ENABLE_BOOTSTRAP=ON -DLLVM_ENABLE_PROJECTS=clang -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD=X86 -G"Ninja" -DLLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS="-n --verbose"

When ninja builds stage-2 it receives the additional -n and --verbose options.

The build_tool_args passed to BUILD_COMMAND looks like:

'--;-n;--verbose'

[russell.gallop]

Resign as reviewer as I work with Carlos (and am not familiar enough with the details of this area).

[andrewng]

Resign as reviewer as I also work with Carlos and helped to author this patch.

[phosek]

LGTM

[CarlosAlbertoEnciso]

In D131665#3740037, @phosek wrote:

LGTM

@phosek Thanks for your review.

clang/CMakeLists.txt

if(DEFINED ${variableName})
       else()
         string(REPLACE ";" "|" value "${${variableName}}")
       endif()
       list(APPEND PASSTHROUGH_VARIABLES
         -D${variableName}=${value})
     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}
     SOURCE_DIR ${CMAKE_SOURCE_DIR}
     STAMP_DIR ${STAMP_DIR}
     BINARY_DIR ${BINARY_DIR}
     EXCLUDE_FROM_ALL 1
     CMAKE_ARGS
                 # We shouldn't need to set this here, but INSTALL_DIR doesn't
                 # seem to work, so instead I'm passing this through
                 -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
                 ${PASSTHROUGH_VARIABLES}
                 ${CLANG_BOOTSTRAP_CMAKE_ARGS}
                  -DCLANG_STAGE=${NEXT_CLANG_STAGE}
                 ${COMPILER_OPTIONS}
                 ${${CLANG_STAGE}_TABLEGEN}
                 ${LTO_LIBRARY} ${verbose} ${PGO_OPT}
                 ${${CLANG_STAGE}_LINKER}
                 ${${CLANG_STAGE}_AR}
                 ${${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
     USES_TERMINAL_BUILD 1
     USES_TERMINAL_INSTALL 1
     LIST_SEPARATOR |
     )