ClangCodeModel: Prefer ui headers from ExtraCompilers

... to the ones from the actual build. The former are generally more up-
to-date.

Fixes: QTCREATORBUG-25937
Change-Id: I20859de9816457c340d9e1ec6a3008c536537d8b
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/clangcodemodel/clangutils.cpp

@@ -100,8 +100,10 @@ private:
     void addDummyUiHeaderOnDiskIncludePath()
     {
         const QString path = ClangModelManagerSupport::instance()->dummyUiHeaderOnDiskDirPath();
-        if (!path.isEmpty())
+        if (!path.isEmpty()) {
-            add({"-I", QDir::toNativeSeparators(path)});
+            prepend(QDir::toNativeSeparators(path));
+            prepend("-I");
+        }
     }
 };
 

src/plugins/cpptools/compileroptionsbuilder.cpp

@@ -165,6 +165,11 @@ void CompilerOptionsBuilder::add(const QString &arg, bool gccOnlyOption)
     add(QStringList{arg}, gccOnlyOption);
 }
 
+void CompilerOptionsBuilder::prepend(const QString &arg)
+{
+    m_options.prepend(arg);
+}
+
 void CompilerOptionsBuilder::add(const QStringList &args, bool gccOnlyOptions)
 {
     m_options.append((gccOnlyOptions && isClStyle()) ? clangArgsForCl(args) : args);

src/plugins/cpptools/compileroptionsbuilder.h

@@ -85,6 +85,7 @@ public:
 
     // Add custom options
     void add(const QString &arg, bool gccOnlyOption = false);
+    void prepend(const QString &arg);
     void add(const QStringList &args, bool gccOnlyOptions = false);
     virtual void addExtraOptions() {}