Add support for MSVC cl-compatible command-line switches.

Patch from Paul Redmond with test by me.

iwyu_driver.cc

@@ -179,7 +179,12 @@ CompilerInstance* CreateCompilerInstance(int argc, const char **argv) {
   // FIXME: This is a hack to try to force the driver to do something we can
   // recognize. We need to extend the driver library to support this use model
   // (basically, exactly one input, and the operation mode is hard wired).
-  args.push_back("-fsyntax-only");
+  driver.ParseDriverMode(args);
+  if (driver.IsCLMode())
+    args.push_back("/Zs");
+  else
+    args.push_back("-fsyntax-only");
+
   unique_ptr<Compilation> compilation(driver.BuildCompilation(args));
   if (!compilation)
     return NULL;

run_iwyu_tests.py

@@ -81,6 +81,7 @@ class OneIwyuTest(unittest.TestCase):
     clang_flags_map = {
       'alias_template.cc': ['-std=c++11'],
       'auto_type_within_template.cc': ['-std=c++11'],
+      'clmode.cc': ['--driver-mode=cl', '/C', '/Os', '/W2'],
       'conversion_ctor.cc': ['-std=c++11'],
       'deleted_implicit.cc' : ['-std=c++11'],
       'lambda_fwd_decl.cc': ['-std=c++11'],

tests/cxx/clmode.cc

@@ -0,0 +1,31 @@
+//===--- clmode.cc - test input file for iwyu -----------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// This test will be executed with --driver-mode=cl and some MSVC-shaped
+// flags to ensure we can run IWYU with MSVC-compatible command-line switches.
+
+#include "tests/cxx/direct.h"
+
+// This use isn't really important, we just want to make sure IWYU does
+// something reasonable even in CL driver mode.
+// IWYU: IndirectClass is...*indirect.h
+IndirectClass random_use;
+
+/**** IWYU_SUMMARY
+
+tests/cxx/clmode.cc should add these lines:
+#include "tests/cxx/indirect.h"
+
+tests/cxx/clmode.cc should remove these lines:
+- #include "tests/cxx/direct.h"  // lines XX-XX
+
+The full include-list for tests/cxx/clmode.cc:
+#include "tests/cxx/indirect.h"  // for IndirectClass
+
+***** IWYU_SUMMARY */