CppTools: Improve clang diagnostic configuration UI

...by clarifying the widget hierarchy. Ensure that the combo box at the
top is perceived as the main widget:

* Remove the "Configuration to use:" label so the combo box gets more
  horizontal space and thus dominance. Also, rename the group box
  header/label from "Clang Diagnostic Warnigns" to "Clang Diagnostic
  Configuration".
* Move the command line text edit below the combo box into its own tab
  page to clarify the relation to the tidy/clazy tabs.

Change-Id: I97a0785678e33b94746046d07a911422b1469890
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/cpptools/cppcodemodelsettingspage.ui

@@ -106,7 +106,7 @@
       </sizepolicy>
      </property>
      <property name="title">
-      <string>Clang Code Model Warnings</string>
+      <string>Clang Diagnostics</string>
      </property>
      <property name="checkable">
       <bool>false</bool>
@@ -114,19 +114,6 @@
      <layout class="QVBoxLayout" name="verticalLayout_3"/>
     </widget>
    </item>
-   <item>
-    <spacer name="verticalSpacer">
-     <property name="orientation">
-      <enum>Qt::Vertical</enum>
-     </property>
-     <property name="sizeHint" stdset="0">
-      <size>
-       <width>20</width>
-       <height>151</height>
-      </size>
-     </property>
-    </spacer>
-   </item>
   </layout>
  </widget>
  <resources/>