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/pp-engine.cpp

@@ -1205,6 +1205,8 @@ void Preprocessor::processDefine(TokenIterator firstToken, TokenIterator lastTok
     macro.setFileName(env->currentFile);
     macro.setLine(env->currentLine);
     macro.setName(tokenText(*tk));
+    macro.setOffset(firstToken->offset);
+    macro.setLength(endOfToken(lastToken[- 1]) - startOfToken(*firstToken));
     ++tk; // skip T_IDENTIFIER
 
     if (tk->is(T_LPAREN) && ! tk->f.whitespace) {