ClangCodeModel: Add experimental clangd support

If the user has enabled clangd (default is off), we start up one instance
per project when it is opened/changed (including build config switches),
and trigger background indexing.
So far, the index is used to provide results for locators and "Find
Usages".
Per-document functionality such as semantic highlighting and completion
is still provided by libclang.

Change-Id: I12532fca1b9c6278baab560e7238cba6189cde9f
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cppcodemodelsettingspage.ui

@@ -6,7 +6,7 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>647</width>
+    <width>697</width>
     <height>440</height>
    </rect>
   </property>
@@ -80,6 +80,33 @@
         </item>
        </layout>
       </item>
+      <item>
+       <layout class="QHBoxLayout" name="horizontalLayout_3">
+        <item>
+         <widget class="QCheckBox" name="clangdCheckBox">
+          <property name="text">
+           <string>Use clangd (EXPERIMENTAL)</string>
+          </property>
+         </widget>
+        </item>
+        <item>
+         <widget class="Utils::PathChooser" name="clangdChooser"/>
+        </item>
+        <item>
+         <spacer name="horizontalSpacer_3">
+          <property name="orientation">
+           <enum>Qt::Horizontal</enum>
+          </property>
+          <property name="sizeHint" stdset="0">
+           <size>
+            <width>40</width>
+            <height>20</height>
+           </size>
+          </property>
+         </spacer>
+        </item>
+       </layout>
+      </item>
      </layout>
     </widget>
    </item>
@@ -147,6 +174,11 @@
    <extends>QWidget</extends>
    <header>cpptools/clangdiagnosticconfigsselectionwidget.h</header>
   </customwidget>
+  <customwidget>
+   <class>Utils::PathChooser</class>
+   <extends>QLineEdit</extends>
+   <header location="global">utils/pathchooser.h</header>
+  </customwidget>
  </customwidgets>
  <resources/>
  <connections/>