ClangCodeModel: Fix some clazy level0 warnings

Use QString instead of an empty QStringLiteral
  [-Wclazy-empty-qstringliteral]

Use the static QFileInfo::exists() instead. It's documented to be
  faster. [-Wclazy-qfileinfo-exists]

Unused CppTools::ClangDiagnosticConfigs
  [-Wclazy-unused-non-trivial-variable]

Change-Id: Ia4098d1191d6fcfc6e0774f71c39acdea3f0f36c
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangcodemodel/clangbackendcommunicator.cpp

@@ -63,9 +63,7 @@ enum { backEndStartTimeOutInMs = 10000 };
 
 static QString backendProcessPath()
 {
-    return Core::ICore::libexecPath()
-            + QStringLiteral("/clangbackend")
-            + QStringLiteral(QTC_HOST_EXE_SUFFIX);
+    return Core::ICore::libexecPath() + "/clangbackend" + QTC_HOST_EXE_SUFFIX;
 }
 
 namespace ClangCodeModel {
@@ -121,7 +119,7 @@ BackendCommunicator::~BackendCommunicator()
 void BackendCommunicator::initializeBackend()
 {
     const QString clangBackEndProcessPath = backendProcessPath();
-    if (!QFileInfo(clangBackEndProcessPath).exists()) {
+    if (!QFileInfo::exists(clangBackEndProcessPath)) {
         logExecutableDoesNotExist();
         return;
     }

src/plugins/clangcodemodel/clangtextmark.cpp

@@ -133,7 +133,6 @@ void disableDiagnosticInCurrentProjectConfig(const ClangBackEnd::DiagnosticConta
         currentConfigId = globalSettings->clangDiagnosticConfigId();
 
     // Get config
-    const ClangDiagnosticConfigs originalConfigs = globalSettings->clangCustomDiagnosticConfigs();
     ClangDiagnosticConfigsModel configsModel(globalSettings->clangCustomDiagnosticConfigs());
     QTC_ASSERT(configsModel.hasConfigWithId(currentConfigId), return );
     ClangDiagnosticConfig config = configsModel.configWithId(currentConfigId);