clang-cl: Support /link option and set target to win32

This adds support for the /link option, which forwards
subsequent arguments to the linker.

The test for this will only work when targetting win32.
Since that's the only target where clang-cl makes sense,
use that target by default.

Differential Revision: http://llvm-reviews.chandlerc.com/D1388

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188331 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index 2ae23ae..5dd6fa1 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -6590,12 +6590,14 @@
   CmdArgs.push_back("-nologo");
 
   Args.AddAllArgValues(CmdArgs, options::OPT_l);
+  Args.AddAllArgValues(CmdArgs, options::OPT__SLASH_link);
 
   // Add filenames immediately.
   for (InputInfoList::const_iterator
        it = Inputs.begin(), ie = Inputs.end(); it != ie; ++it) {
     if (it->isFilename())
       CmdArgs.push_back(it->getFilename());
+    // FIXME: Forward -Wl, etc.
   }
 
   const char *Exec =