Task-number: QTCREATORBUG-1776

Previously, it would still fail if a forward declaration and declaration
for a class were in the same file.

Task-number: QTCREATORBUG-20

If it encounters a forward declaration, it tries to find the class
declaration globally now.

Task-number: QTCREATORBUG-20

It introduced at least one new crash to do with recovering from parsing
errors.

Reviewed-by: Roberto Raggi

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

Encapsulate the details.

Now, the ASTVisitor constructor takes a valid reference to a TranslationUnit.

Done with Erik Verbruggen

Ensure the code model receives file names with clean paths ('/')
and fix paths in the relevant places of the code model. Pass on clean
paths from Cpp reference find and display them correctly in the search
window tooltip.

Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

updateModifiedFiles ckecks the timestamp and reparses all the modified files in the current snapshot.

In particular macros that are only checked for definition or are
expanded during the evaluation of an #if or #elif directive are now also
added to the list available through Document::macroUses().

The names of undefined macros that are interesting (because they're used
in an #ifdef or a defined(...)) are now available through
Document::undefinedMacroUses().

Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

Snapshot::dependsOn(fn) returns the files in the snapshot that depends on fn.

Reviewed by Thorbjørn Lindeijer