ProjectExplorer: Parse macros when clang-cl exits with non-zero code

clang-cl still provides the list of macros even if it reports
some option being not supported. Try to parse the output
even in such cases.

Change-Id: I4fa070c7c465166fdf70c03944b2713478acba13
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/msvctoolchain.cpp

@@ -1127,7 +1127,8 @@ Macros ClangClToolChain::msvcPredefinedMacros(const QStringList cxxflags,
                                                                  arguments);
     if (response.result != Utils::SynchronousProcessResponse::Finished ||
             response.exitCode != 0) {
-        return {};
+        // Show the warning but still parse the output.
+        QTC_CHECK(false && "clang-cl exited with non-zero code.");
     }
 
     return Macro::toMacros(response.allRawOutput());