[llvm-strip] Expose --discard-all option

Expose objcopy's --discard-all option in llvm-strip.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D47750


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334131 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed