Enhance data stored for macros and macro uses.

In preparation for finding macro uses.

* Macro: add offset and length
* MacroUse: add line
* Document: add convenience functions for finding a macro definition, use
  or undefined use at a given location.

Reviewed-by: Erik Verbruggen

src/libs/cplusplus/Macro.cpp

@@ -54,6 +54,8 @@ Macro::Macro()
     : _next(0),
       _hashcode(0),
       _line(0),
+      _offset(0),
+      _length(0),
       _state(0)
 { }