Clang: Minimize reindexing

We optimal indexer is only reindexing if the index would be changed. This
patch is a step in that direction. We only reindex now if the file or
project has changed. It fixes some typos too.

Task-number: QTCREATORBUG-21150
Change-Id: I6ea1c13282fbcd70253b9b2939aed37580dbd160
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/cpptools/compileroptionsbuilder.cpp

@@ -455,6 +455,8 @@ void CompilerOptionsBuilder::addLanguageVersionAndExtensions()
     case LanguageVersion::CXX2a:
         option = (gnuExtensions ? QLatin1String("-std=gnu++2a") : QLatin1String("-std=c++2a"));
         break;
+    case LanguageVersion::None:
+        break;
     }
 
     add(option, /*gccOnlyOption=*/true);