Misc enhancements to LTO:

  1. Add some helper classes for partitions. They are designed in a
     way such that the top-level LTO driver will not see much difference
     with or without partitioning.

  2. Introduce work-dir. Now all intermediate files generated during
     LTO phases will be saved under work-dir. User can specify the workdir
     via -lto-workdir=/path/to/dir. By default the work-dir will be
     erased before linker exit. To keep the workdir, do -lto-keep, or -lto-keep=1.

    TODO: Erase the workdir, if the linker exit prematurely.
      We are currently not able to remove directory on signal. The support
      routines simply ignore directory.

  3. Add one new API lto_codegen_get_files_need_remove().
     Linker and LTO plugin will communicate via this API about which files
    (including directories) need to removed before linker exit.