Reland r341390 clang-cl: Pass /Brepro to linker if it was passed to the compiler

The test was missing  '--' on mac as pointed out by -Wslash-u-filename:
<stdin>:5:69: note: possible intended match here
clang: warning: '/Users/thakis/src/llvm-mono/clang/test/Driver/msvc-link.c' treated as the '/U' option [-Wslash-u-filename]

Differential Revision: https://reviews.llvm.org/D51635

llvm-svn: 341654