Clang: Add semantic C++ operator-token styling

We used to style overloaded operators in the same way
as C++'s built-in operators. There was no way to tell
if a + token would call a operator+() function or not.

Now, if an operator is overloaded (redefined),
we give it the "Overloaded Operator"-mixin so users
can style it differently.

Note: Calls to overloaded 'new' and 'delete' are not
highlighted by "Overloaded Operator". This is because
clang today always maps these to CXCursor_CXXNewExpr
and CXCursor_CXXDeleteExpr with cursor.spelling == ""
(empty string). So there is no (?) quick way for us
to tell if a new/delete-token was overloaded or not.

After follow-ups, follow symbol will work for operator
overload usages in current translation unit.
Commit is appended by Ivan Donchevskii.

Task-number: QTCREATORBUG-19659
Change-Id: I157855d482a61ad2059642a1ee982089fcb7d312
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangbackend/source/clangstring.h

@@ -91,6 +91,11 @@ public:
         return !isNull() && std::strlen(cString()) > 0;
     }
 
+    bool startsWith(const char* str) const
+    {
+        return std::strncmp(cString(), str, strlen(str)) == 0;
+    }
+
     friend bool operator==(const ClangString &first, const ClangString &second)
     {
         return std::strcmp(first.cString(), second.cString()) == 0;
@@ -107,6 +112,7 @@ public:
     {
         return second == first;
     }
+
     template<typename Type,
              typename = typename std::enable_if<std::is_pointer<Type>::value>::type
              >
@@ -123,6 +129,39 @@ public:
         return second == first;
     }
 
+    friend bool operator!=(const ClangString &first, const ClangString &second)
+    {
+        return !(first == second);
+    }
+
+    template<std::size_t Size>
+    friend bool operator!=(const ClangString &first, const char(&second)[Size])
+    {
+        return !(first == second);
+    }
+
+    template<std::size_t Size>
+    friend bool operator!=(const char(&first)[Size], const ClangString &second)
+    {
+        return second != first;
+    }
+
+    template<typename Type,
+             typename = typename std::enable_if<std::is_pointer<Type>::value>::type
+             >
+    friend bool operator!=(const ClangString &first, Type second)
+    {
+        return !(first == second);
+    }
+
+    template<typename Type,
+             typename = typename std::enable_if<std::is_pointer<Type>::value>::type
+             >
+    friend bool operator!=(Type first, const ClangString &second)
+    {
+        return !(first == second);
+    }
+
     friend std::ostream &operator<<(std::ostream &os, const ClangString &string)
     {
         return os << string.cString();