Clang: Allow to forward warnings flags from build system

Change-Id: I47ebb1ce4f3b5544408eb1d0f891ed5090394282
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangtools/clangtoolruncontrol.cpp

@@ -195,6 +195,7 @@ static AnalyzeUnits toAnalyzeUnits(const FileInfos &fileInfos)
                                               UseSystemHeader::No,
                                               UseTweakedHeaderPaths::Yes,
                                               UseLanguageDefines::No,
+                                              UseBuildSystemWarnings::No,
                                               QString(CLANG_VERSION),
                                               QString(CLANG_RESOURCE_DIR));
         QStringList arguments = extraClangToolsPrependOptions();