When --build-id=tree is selected, gold would schedule a set of
tasks to run to compute md5 hashes in parallel on chunks of the
file. The scheduling was done before the
Write_after_input_sections_task ran, so if we are compressing
debug sections, the output file will change size and be remapped
to a new address, sometimes causing the build id computation to
crash, but even when it doesn't crash, it wouldn't include the
debug information in the hash computation.

This patch delays the scheduling of the md5 tasks until after
Write_after_input_sections_task.

gold/
        PR gold/17819
        * gold.cc (queue_final_tasks): When --build-id=tree, queue a
        separate task to schedule the build id computation.
        * layout.cc (Hash_task::Hash_task): Remove build_id_blocker,
        add Output_file and offset.
        (Hash_task::run): Get and release the input views.
        (Hash_task::is_runnable): Always return NULL (always runnable).
        (Layout::queue_build_id_tasks): Remove.
        (Layout::write_build_id): Add array_of_hashes and size_of_hashes
        parameters; use them instead of class members.
        (Build_id_task_runner::run): New function.
        (Close_task_runner::run): Pass array_of_hashes and size_of_hashes
        to write_build_id.
        * layout.h (Layout::queue_build_id_tasks): Remove.
        (Layout::write_build_id): Add array_of_hashes and size_of_hashes
        parameters.
        (Layout::array_of_hashes_): Remove.
        (Layout::size_of_array_of_hashes_): Remove.
        (Layout::input_view_): Remove.
        (Build_id_task_runner): New class.
        (Close_task_runner::Close_task_runner): Add array_of_hashes and
        size_of_hashes parameters.
        (Close_task_runner::array_of_hashes_): New data member.
        (Close_task_runner::size_of_hashes_): New data member.
        * testsuite/Makefile.am
        (flagstest_compress_debug_sections_and_build_id_tree): New test.
        * testsuite/Makefile.in: Regenerate.