clang-cl: fix passing optimization level to cl.exe in /fallback mode

We were previously mostly passing it through, but -O0 and -O3 are not valid
options to cl.exe.

We should translate -O0 to /Od and -O3 to /Ox. -O{1,2,s} get passed through.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191323 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index 5b96120..e5f2681 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -6712,7 +6712,19 @@
   // These are spelled the same way in clang and cl.exe,.
   Args.AddAllArgs(CmdArgs, options::OPT_D, options::OPT_U);
   Args.AddAllArgs(CmdArgs, options::OPT_I);
-  Args.AddLastArg(CmdArgs, options::OPT_O, options::OPT_O0);
+
+  // Optimization level.
+  if (Arg *A = Args.getLastArg(options::OPT_O, options::OPT_O0)) {
+    if (A->getOption().getID() == options::OPT_O0) {
+      CmdArgs.push_back("/Od");
+    } else {
+      StringRef OptLevel = A->getValue();
+      if (OptLevel == "1" || OptLevel == "2" || OptLevel == "s")
+        A->render(Args, CmdArgs);
+      else if (OptLevel == "3")
+        CmdArgs.push_back("/Ox");
+    }
+  }
 
   // Flags for which clang-cl have an alias.
   // FIXME: How can we ensure this stays in sync with relevant clang-cl options?