Clang: Make ProjectPartId StringCache friendly

Change-Id: I53fdeb87ac67ca2c2c7509bf725566298f3b2963
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/clangsupport/projectpartid.h

@@ -31,6 +31,15 @@ namespace ClangBackEnd {
 
 class ProjectPartId
 {
+    template<typename StringType,
+             typename StringViewType,
+             typename IndexType,
+             typename Mutex,
+             typename Compare,
+             Compare compare,
+             typename CacheEntry>
+    friend class StringCache;
+
 public:
     constexpr ProjectPartId() = default;
 
@@ -47,6 +56,11 @@ public:
         return first.isValid() && first.projectPathId == second.projectPathId;
     }
 
+    friend bool operator==(ProjectPartId first, int second)
+    {
+        return first == ProjectPartId{second};
+    }
+
     friend bool operator!=(ProjectPartId first, ProjectPartId second) { return !(first == second); }
 
     friend bool operator<(ProjectPartId first, ProjectPartId second)
@@ -68,6 +82,10 @@ public:
         return in;
     }
 
+private:
+    operator int() const { return projectPathId; }
+    ProjectPartId operator++() { return ++projectPathId; }
+
 public:
     int projectPathId = -1;
 };