[llvm-strip] Expose --keep-symbol option

Expose --keep-symbol option in llvm-strip.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D47222


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333117 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed