[libcxxabi] Copy headers into build location

[phosek]

Summary

Prior to D120727, the libcxx build was responsible for copying libcxxabi
headers into the right location, both in the build and install trees,
but now it's the responsibility of the libcxxabi build. While the build
already did the right thing for the install tree, it wouldn't copy
headers into the build tree, resulting in errors when trying to use the
just built toolchain as is the case in the runtimes build when building
compiler-rt runtimes.

[paulkirth]

LGTM, given that the patch basically restores the code removed in D120727.

[ldionne]

I think I don't understand why compiler-rt would need those headers to be present. Why is it trying to use libc++/libc++abi headers inside build/? Instead, shouldn't it install the toolchain to a temporary directory and then build against that?

libcxxabi/CMakeLists.txt

if (LIBCXXABI_BUILD_32_BITS)
   message(FATAL_ERROR "LIBCXXABI_BUILD_32_BITS is not supported anymore when building the runtimes, please specify a full triple instead.")
 endif()
 
 option(LIBCXXABI_INCLUDE_TESTS "Generate build targets for the libc++abi unit tests." ${LLVM_INCLUDE_TESTS})
 set(LIBCXXABI_LIBDIR_SUFFIX "${LLVM_LIBDIR_SUFFIX}" CACHE STRING
     "Define suffix of library directory name (32/64)")
 option(LIBCXXABI_INSTALL_HEADERS "Install the libc++abi headers." ON)
 option(LIBCXXABI_INSTALL_LIBRARY "Install the libc++abi library." ON)
-set(LIBCXXABI_INSTALL_INCLUDE_DIR "include/c++/v1" CACHE PATH "Path to install the libc++abi headers at.")
+
+set(LIBCXXABI_INSTALL_INCLUDE_DIR "${CMAKE_INSTALL_INCLUDEDIR}/c++/v1" CACHE PATH "Path to install the libc++abi headers at.")
+
+if(LLVM_LIBRARY_OUTPUT_INTDIR)
+  set(LIBCXXABI_GENERATED_INCLUDE_DIR "${LLVM_BINARY_DIR}/include/c++/v1")
+else()
+  set(LIBCXXABI_GENERATED_INCLUDE_DIR "${CMAKE_BINARY_DIR}/include/c++/v1")
+endif()
 
 # TODO: Remove this after branching for LLVM 15
 if(LIBCXXABI_SYSROOT OR LIBCXXABI_TARGET_TRIPLE OR LIBCXXABI_GCC_TOOLCHAIN)
   message(WARNING "LIBCXXABI_SYSROOT, LIBCXXABI_TARGET_TRIPLE and LIBCXXABI_GCC_TOOLCHAIN are not supported anymore, please use the native CMake equivalents instead")
 endif()
 
 set(LIBCXXABI_LIBCXX_LIBRARY_PATH "" CACHE PATH "The path to libc++ library.")
 set(LIBCXXABI_LIBRARY_VERSION "1.0" CACHE STRING

libcxxabi/include/CMakeLists.txt

 set(files
   __cxxabi_config.h
   cxxabi.h
   )
 
+foreach(f ${files})
+  set(src "${CMAKE_CURRENT_SOURCE_DIR}/${f}")
+  set(dst "${LIBCXXABI_GENERATED_INCLUDE_DIR}/${f}")
+  add_custom_command(OUTPUT ${dst}
+    DEPENDS ${src}
+    COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
+    COMMENT "Copying CXXABI header ${f}")
+  list(APPEND _all_includes "${dst}")
+endforeach()
+
+add_custom_target(generate-cxxabi-headers ALL DEPENDS ${_all_includes})
+
 add_library(cxxabi-headers INTERFACE)
+add_dependencies(cxxabi-headers generate-cxxabi-headers)
 target_include_directories(cxxabi-headers INTERFACE "${CMAKE_CURRENT_SOURCE_DIR}")
 
 if (LIBCXXABI_INSTALL_HEADERS)
   foreach(file ${files})
     get_filename_component(dir ${file} DIRECTORY)
     install(FILES ${file}
       DESTINATION ${LIBCXXABI_INSTALL_INCLUDE_DIR}/${dir}
       COMPONENT cxxabi-headers