[libc] Fix the libc header installation path

[phosek]

Summary

This results in headers being installed in the wrong location.

libc/include/CMakeLists.txt

 target_include_directories(libc-headers SYSTEM INTERFACE ${LIBC_INCLUDE_DIR})
 
 foreach(target IN LISTS all_install_header_targets)
   get_target_property(header_file ${target} HEADER_FILE_PATH)
   if(NOT header_file)
     message(FATAL_ERROR "Installable header file '${target}' does not have the "
                         "HEADER_FILE_PATH property set.")
   endif()
-  file(RELATIVE_PATH relative_path ${LIBC_INCLUDE_BINARY_DIR} ${header_file})
+  file(RELATIVE_PATH relative_path ${LIBC_INCLUDE_DIR} ${header_file})
   get_filename_component(nested_dir ${relative_path} DIRECTORY)
   install(FILES ${header_file}
           DESTINATION ${LIBC_INSTALL_INCLUDE_DIR}/${nested_dir}
           COMPONENT libc-headers)
   # The GPU optionally provides the supported declarations externally so
   # offloading languages like CUDA and OpenMP know what is supported by libc. We
   # install these in the compiler's resource directory at a preset location.
   if(LIBC_TARGET_ARCHITECTURE_IS_GPU AND PACKAGE_VERSION)
     get_target_property(decls_file ${target} DECLS_FILE_PATH)
     if(NOT decls_file)
       continue()
     endif()
     get_clang_resource_dir(resource_dir SUBDIR include)
-    file(RELATIVE_PATH relative_path ${LIBC_INCLUDE_BINARY_DIR} ${decls_file})
+    file(RELATIVE_PATH relative_path ${LIBC_INCLUDE_DIR} ${decls_file})
     get_filename_component(nested_dir ${relative_path} DIRECTORY)
     set(install_dir
         ${CMAKE_INSTALL_PREFIX}/${resource_dir}/llvm_libc_wrappers/${nested_dir})
     install(FILES ${decls_file}
             DESTINATION ${install_dir}
             COMPONENT libc-headers)
   endif()
 endforeach()