[libcxxabi] Copy headers into build location

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.

Differential Revision: https://reviews.llvm.org/D125597

libcxxabi/CMakeLists.txt

@@ -78,7 +78,14 @@ 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)

libcxxabi/include/CMakeLists.txt

@@ -3,7 +3,20 @@ set(files
   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)