[llvm-strip] Check "strip" with StringRef::contains instead of ends_with

Summary: If argv[0] is version suffixed, e.g. llvm-strip-7, this will still work.

Reviewers: rupprecht, jhenderson, alexshap, jakehehrlich

Reviewed By: rupprecht

Subscribers: alexshap, jakehehrlich, llvm-commits

Differential Revision: https://reviews.llvm.org/D54193

llvm-svn: 346292