[CMake] Rename cxx_headers back to cxx-headers.

[ab]

Summary

r334477 renamed the cxx-headers target to cxx_headers, but various pieces sort-of expect the target names to match the component (e.g., LLVM_DISTRIBUTION_COMPONENTS in the various bootstrap caches, which, via some magic foreign to me, seems to expect cxx-headers, install-cxx-headers, and install-cxx-headers-stripped to exist)

We're still finding places that need to be changed, so, unless there was a reason for that change, revert back to cxx-headers (which, anecdotally, matches "clang-headers" for the clang lib/Headers/)

Otherwise, perhaps we should have target "aliases" (it seems like that's the reason why there are both *libcxx-headers* and *cxx-headers* variants of a few targets)

[phosek]

LGTM

[phosek]

It's not obvious to me how anything could depend on cxx-headers target (not the component) since that target did not exist until r334468 which has landed only recently (just before r334477). The motivation behind this change was just consistency since all other targets in libc++ use underscores rather than dashes, so reverting that change is fine with me if it's breaking you.

include/CMakeLists.txt

if (LIBCXX_NEEDS_SITE_CONFIG)
     set(dst ${output_dir}/__config)
     add_custom_command(OUTPUT ${dst}
         DEPENDS ${src} ${generated_config_deps}
         COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
         COMMENT "Copying CXX __config")
     list(APPEND out_files ${dst})
   endif()
 
-  add_custom_target(cxx_headers ALL DEPENDS ${out_files} ${LIBCXX_CXX_ABI_HEADER_TARGET})
+  add_custom_target(cxx-headers ALL DEPENDS ${out_files} ${LIBCXX_CXX_ABI_HEADER_TARGET})
 else()
-  add_custom_target(cxx_headers)
+  add_custom_target(cxx-headers)
 endif()
-set_target_properties(cxx_headers PROPERTIES FOLDER "Misc")
+set_target_properties(cxx-headers PROPERTIES FOLDER "Misc")
 
 if (LIBCXX_INSTALL_HEADERS)
   foreach(file ${files})
     get_filename_component(dir ${file} DIRECTORY)
     install(FILES ${file}
       DESTINATION ${LIBCXX_INSTALL_PREFIX}include/c++/v1/${dir}
       COMPONENT cxx-headers
       PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
     )
   endforeach()
 
   if (LIBCXX_NEEDS_SITE_CONFIG)
     # Install the generated header as __config.
     install(FILES ${LIBCXX_BINARY_DIR}/__generated_config
       DESTINATION ${LIBCXX_INSTALL_PREFIX}include/c++/v1
       PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
       RENAME __config
       COMPONENT cxx-headers)
   endif()
 
   if (NOT CMAKE_CONFIGURATION_TYPES)
     add_custom_target(install-cxx-headers
-                      DEPENDS cxx_headers ${generated_config_deps}
+                      DEPENDS cxx-headers ${generated_config_deps}
                       COMMAND "${CMAKE_COMMAND}"
                               -DCMAKE_INSTALL_COMPONENT=cxx-headers
                               -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
     # Stripping is a no-op for headers
     add_custom_target(install-cxx-headers-stripped DEPENDS install-cxx-headers)
 
     add_custom_target(install-libcxx-headers DEPENDS install-cxx-headers)
     add_custom_target(install-libcxx-headers-stripped DEPENDS install-cxx-headers-stripped)
   endif()
 endif()

lib/CMakeLists.txt

ARGS
       "${MERGE_ARCHIVES_ABI_TARGET}"
       "${MERGE_ARCHIVES_SEARCH_PATHS}"
     WORKING_DIRECTORY ${LIBCXX_BUILD_DIR}
     )
   endif()
 endif()
 
 # Add a meta-target for both libraries.
-add_custom_target(cxx DEPENDS cxx_headers ${LIBCXX_TARGETS})
+add_custom_target(cxx DEPENDS cxx-headers ${LIBCXX_TARGETS})
 
 if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
   file(GLOB LIBCXX_EXPERIMENTAL_SOURCES ../src/experimental/*.cpp)
   if (LIBCXX_ENABLE_FILESYSTEM)
     file(GLOB LIBCXX_FILESYSTEM_SOURCES ../src/experimental/filesystem/*.cpp)
   endif()
   add_library(cxx_experimental STATIC ${LIBCXX_EXPERIMENTAL_SOURCES} ${LIBCXX_FILESYSTEM_SOURCES})
   if (LIBCXX_ENABLE_SHARED)