[llvm-strip]Add --no-strip-all to disable --strip-all behaviour (including default stripping)

If certain switches are not specified, llvm-strip behaves as if
--strip-all were specified. This means that for testing, when we don't
want the stripping behaviour, we have to specify one of these switches,
which can be confusing. This change adds --no-strip-all to allow an
alternative way of suppressing the default stripping, in a less
confusing manner.

Reviewed by: jakehehrlich, MaskRay

Differential Revision: https://reviews.llvm.org/D61377

llvm-svn: 359781