Clang: Fix the include paths order for libclang

Clang headers should always come first. After we
removed '-isystem' from other paths it was still
used for clang headers which put it in the end of list.

This fix returns the paths their original order.

Change-Id: I0a4ab450a5303e2536e60200f479cc19f6f55e99
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/cpptools/compileroptionsbuilder.cpp

@@ -513,7 +513,7 @@ void CompilerOptionsBuilder::addPredefinedHeaderPathsOptions()
 void CompilerOptionsBuilder::addClangIncludeFolder()
 {
     QTC_CHECK(!m_clangVersion.isEmpty());
-    add(SYSTEM_INCLUDE_PREFIX);
+    add(includeDirOption());
     add(clangIncludeDirectory(m_clangVersion, m_clangResourceDirectory));
 }