Clang: Check that file kind matches the language version

It was checked that pure C header and source files had proper
language version but not another way around.

Change-Id: I1eaf41f342731bdbe30ee73e81144334d0f15d19
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/compileroptionsbuilder.cpp

@@ -65,6 +65,11 @@ QStringList CompilerOptionsBuilder::build(CppTools::ProjectFile::Kind fileKind,
                    return QStringList(););
     }
 
+    if (fileKind == ProjectFile::CXXHeader || fileKind == ProjectFile::CXXSource) {
+        QTC_ASSERT(m_projectPart.languageVersion > ProjectExplorer::LanguageVersion::LatestC,
+                   return QStringList(););
+    }
+
     add("-c");
 
     addWordWidth();