[Driver] Fix assertion failure when -fprofile-generate -fcs-profile-generate are used together

[MaskRay]

Summary

If conflicting -fprofile-generate -fcs-profile-generate are used together,
there is currently an assertion failure. Fix the failure.

Also add some driver tests.

[MaskRay]

.

[xur]

looks good to me. Thanks for working on this.

[MaskRay]

Fix test on Windows

clang/lib/Driver/ToolChains/Clang.cpp

static void addPGOAndCoverageFlags(const ToolChain &TC, Compilation &C,
   if (PGOGenerateArg && ProfileUseArg)
     D.Diag(diag::err_drv_argument_not_allowed_with)
         << ProfileUseArg->getSpelling() << PGOGenerateArg->getSpelling();
 
   if (ProfileGenerateArg && ProfileUseArg)
     D.Diag(diag::err_drv_argument_not_allowed_with)
         << ProfileGenerateArg->getSpelling() << ProfileUseArg->getSpelling();
 
-  if (CSPGOGenerateArg && PGOGenerateArg)
+  if (CSPGOGenerateArg && PGOGenerateArg) {
     D.Diag(diag::err_drv_argument_not_allowed_with)
         << CSPGOGenerateArg->getSpelling() << PGOGenerateArg->getSpelling();
+    PGOGenerateArg = nullptr;
+  }
 
   if (ProfileGenerateArg) {
     if (ProfileGenerateArg->getOption().matches(
             options::OPT_fprofile_instr_generate_EQ))
       CmdArgs.push_back(Args.MakeArgString(Twine("-fprofile-instrument-path=") +
                                            ProfileGenerateArg->getValue()));
     // The default is to use Clang Instrumentation.
     CmdArgs.push_back("-fprofile-instrument=clang");

clang/test/Driver/fcs-profile-generate.c

+// RUN: %clang -### -c -fprofile-use=a.profdata -fcs-profile-generate %s 2>&1 | FileCheck %s
+// CHECK:      "-fprofile-instrument=csllvm"
+// CHECK-NOT:  "-fprofile-instrument-path=
+// CHECK-SAME: "-fprofile-instrument-use-path=a.profdata"
+
+// RUN: %clang -### -c -fprofile-use=a.profdata -fcs-profile-generate=dir %s 2>&1 | FileCheck %s --check-prefix=CHECK1
+// CHECK1: "-fprofile-instrument=csllvm"{{.*}} "-fprofile-instrument-path=dir{{/|\\\\}}default_%m.profraw" "-fprofile-instrument-use-path=a.profdata"
+
+/// Degradation case. This usage does not make much sense.
+// RUN: %clang -### -c -fcs-profile-generate %s 2>&1 | FileCheck %s --check-prefix=NOUSE
+// NOUSE:     "-fprofile-instrument=csllvm"
+// NOUSE-NOT: "-fprofile-instrument-path=
+
+// RUN: %clang -### -c -fprofile-generate -fcs-profile-generate %s 2>&1 | FileCheck %s --check-prefix=CONFLICT
+// CONFLICT: error: invalid argument '-fcs-profile-generate' not allowed with '-fprofile-generate'