Add system include path to HeaderPath and merge ProjectPartHeaderPath

System include paths are appended after other includes by the compiler. So
we should set them as system includes and not as normal includes. Otherwise
we change the include order. Headers in system include paths are not
cluttering the screen with unwanted warning and by the way improve
performance too.

ProjectPartHeaderPath was a dopperganger of HeaderPath, so we merged them.

Change-Id: I7c394b4098b697de79761499ffcd5913cc02d652
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cpptools/compileroptionsbuilder.cpp

@@ -198,11 +198,11 @@ void CompilerOptionsBuilder::enableExceptions()
 
 void CompilerOptionsBuilder::addHeaderPathOptions()
 {
-    typedef ProjectPartHeaderPath HeaderPath;
+    using ProjectExplorer::IncludePathType;
 
     QStringList result;
 
-    foreach (const HeaderPath &headerPath , m_projectPart.headerPaths) {
+    for (const ProjectExplorer::HeaderPath &headerPath : qAsConst(m_projectPart.headerPaths)) {
         if (headerPath.path.isEmpty())
             continue;
 
@@ -212,12 +212,15 @@ void CompilerOptionsBuilder::addHeaderPathOptions()
         QString prefix;
         Utils::FileName path;
         switch (headerPath.type) {
-        case HeaderPath::FrameworkPath:
+        case IncludePathType::Framework:
             prefix = QLatin1String("-F");
             break;
+        case IncludePathType::System:
+            prefix = "-isystem";
+            break;
         default: // This shouldn't happen, but let's be nice..:
             // intentional fall-through:
-        case HeaderPath::IncludePath:
+        case IncludePathType::User:
             prefix = includeDirOptionForPath(headerPath.path);
             break;
         }