Add --strip-all option back to llvm-strip.

Summary:
This option appears to have been dropped as part of the refactoring in
r331663. Unfortunately, if we want to use llvm-strip as a drop-in
replacement for strip, this option should still be available.

Reviewers: alexshap

Reviewed By: alexshap

Subscribers: meikeb, kongyi, chh, jakehehrlich, llvm-commits, pirama

Differential Revision: https://reviews.llvm.org/D49226

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336921 91177308-0d34-0410-b5e6-96231b3b80d8