[CMake] Pass doc-related LLVM variables to external projects

[hawkinsw]

Summary

When generating the configuration arguments for external projects,
some of the external-project-specific CMake variables only work
in combination with CMake variables from LLVM itself. In particular,
the some of the doc-related CMake variables in libcxx depend on
LLVM_ENABLE_SPHINX. Pass

LLVM_ENABLE_SPHINX
LLVM_BUILD_DOCS

to external projects.

[hawkinsw]

Migrated to GitHub: https://github.com/llvm/llvm-project/pull/65531

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

CMAKE_ARGS ${${nameCanon}_CMAKE_ARGS}
                -DLLVM_HAVE_LINK_VERSION_SCRIPT=${LLVM_HAVE_LINK_VERSION_SCRIPT}
                -DLLVM_USE_RELATIVE_PATHS_IN_DEBUG_INFO=${LLVM_USE_RELATIVE_PATHS_IN_DEBUG_INFO}
                -DLLVM_USE_RELATIVE_PATHS_IN_FILES=${LLVM_USE_RELATIVE_PATHS_IN_FILES}
                -DLLVM_LIT_ARGS=${LLVM_LIT_ARGS}
                -DLLVM_SOURCE_PREFIX=${LLVM_SOURCE_PREFIX}
                -DPACKAGE_VERSION=${PACKAGE_VERSION}
                -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
                -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
+               -DLLVM_ENABLE_SPHINX=${LLVM_ENABLE_SPHINX}
+               -DLLVM_BUILD_DOCS=${LLVM_BUILD_DOCS}
                -DCMAKE_EXPORT_COMPILE_COMMANDS=1
                ${cmake_args}
                ${PASSTHROUGH_VARIABLES}
     INSTALL_COMMAND ""
     STEP_TARGETS configure build
     BUILD_ALWAYS 1
     USES_TERMINAL_CONFIGURE 1
     USES_TERMINAL_BUILD 1