[libc++][C++20 modules] Fixes CMake include paths.

[Mordante]

Summary

When the modules are used in a bootstrap build the paths
${LIBCXX_GENERATED_INCLUDE_DIR} and ${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}
have a different value and both are needed to build modules.

[Mordante]

For testing purposes this patch has been changed to a GitHub PR https://github.com/llvm/llvm-project/pull/65079

libcxx/modules/CMakeLists.txt

set(LIBCXX_MODULE_STD_SOURCES
   std/vector.inc
   std/version.inc
 )
 
 # TODO MODULES the CMakeLists.txt in the install directory is only temporary
 # When that is removed the configured file can use the substitution
 # LIBCXX_GENERATED_INCLUDE_TARGET_DIR avoiding this set.
 # Also clean up the parts needed to generate the install version.
-set(LIBCXX_CONFIGURED_INCLUDE_DIR ${LIBCXX_GENERATED_INCLUDE_TARGET_DIR})
+# - LIBCXX_GENERATED_INCLUDE_DIR contains the libc++ headers
+# - LIBCXX_GENERATED_INCLUDE_TARGET_DIR contains the libc++ site config
+if ("${LIBCXX_GENERATED_INCLUDE_DIR}" STREQUAL "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}")
+  # This typically happens when the target is not installed.
+  set(LIBCXX_CONFIGURED_INCLUDE_DIRS "${LIBCXX_GENERATED_INCLUDE_DIR}")
+else()
+  set(LIBCXX_CONFIGURED_INCLUDE_DIRS
+    "${LIBCXX_GENERATED_INCLUDE_DIR};${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}"
+  )
+endif()
 configure_file(
   "CMakeLists.txt.in"
   "${LIBCXX_GENERATED_MODULE_DIR}/CMakeLists.txt"
   @ONLY
 )
 
 set(LIBCXX_MODULE_STD_INCLUDE_SOURCES)
 foreach(file ${LIBCXX_MODULE_STD_SOURCES})

libcxx/modules/CMakeLists.txt.in

 
 add_library(std)
 target_sources(std
   PUBLIC FILE_SET cxx_modules TYPE CXX_MODULES FILES
     std.cppm
 )
 
 target_compile_definitions(std PRIVATE _LIBCPP_ENABLE_EXPERIMENTAL)
-target_include_directories(std SYSTEM PRIVATE @LIBCXX_CONFIGURED_INCLUDE_DIR@)
+target_include_directories(std SYSTEM PRIVATE @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
 
 target_compile_options(std
   PUBLIC
     -nostdinc++
     -Wno-reserved-module-identifier
     -Wno-reserved-user-defined-literal
     @LIBCXX_COMPILE_FLAGS@
 )
 set_target_properties(std
   PROPERTIES
     OUTPUT_NAME   "c++std"
 )