lld-link: Let LLD_REPRODUCE control /reproduce:, like in ld.lld

Also sync help texts for the option between elf and coff ports.

Decisions:
- Do this even if /lldignoreenv is passed. /reproduce: does not affect
  the main output, and this makes the env var more convenient to use.
  (On the other hand, it's now possible to set this env var and forget
  about it, and all future builds in the same shell will be much slower.
  That's true for ld.lld, but posix shells have an easy way to set an
  env var for a single command; in cmd.exe this is not possible without
  contortions. Then again, lld-link runs in posix shells too.)

Original patch rebased across D68378 and D68381.

Differential Revision: https://reviews.llvm.org/D67707