clang-cl: Pass /Z7 when we fallback to cl with debug info enabled

Clang itself only emits CodeView line tables, so it seems more
consistent to ask cl.exe for the same format.

llvm-svn: 201721
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index c4982d72..532c863 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -7221,6 +7221,8 @@
                                                                    : "/GR-");
   if (Args.hasArg(options::OPT_fsyntax_only))
     CmdArgs.push_back("/Zs");
+  if (Args.hasArg(options::OPT_g_Flag, options::OPT_gline_tables_only))
+    CmdArgs.push_back("/Z7");
 
   std::vector<std::string> Includes = Args.getAllArgValues(options::OPT_include);
   for (size_t I = 0, E = Includes.size(); I != E; ++I)