clang-cl: Ignore /fallback when not actually compiling (PR18456)

For example, don't fall back in /P (preprocess) mode.

llvm-svn: 199153

clang/lib/Driver/Tools.cpp

@@ -3725,7 +3725,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   }
 
   // Finally add the compile command to the compilation.
-  if (Args.hasArg(options::OPT__SLASH_fallback)) {
+  if (Args.hasArg(options::OPT__SLASH_fallback) &&
+      Output.getType() == types::TY_Object) {
     tools::visualstudio::Compile CL(getToolChain());
     Command *CLCommand = CL.GetCommand(C, JA, Output, Inputs, Args,
                                        LinkingOutput);

clang/test/Driver/cl-fallback.c

@@ -39,3 +39,8 @@
 // RUN: %clang_cl /fallback /Ox -### -- %s 2>&1 | FileCheck -check-prefix=Ox %s
 // Ox: cl.exe
 // Ox: "/Ox"
+
+// Only fall back when actually compiling, not for e.g. /P (preprocess).
+// RUN: %clang_cl /fallback /P -### -- %s 2>&1 | FileCheck -check-prefix=P %s
+// P-NOT: ||
+// P-NOT: "cl.exe"