[llvm-rc] Support '--' for delimiting options from input paths

This allows avoiding conflicts between paths that begin with the same
chars as some llvm-rc options (which can be used with either slashes
or dashes).

Differential Revision: https://reviews.llvm.org/D56743

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351305 91177308-0d34-0410-b5e6-96231b3b80d8
22 files changed