Clang: implement followSymbol in TranslationUnit

Follow symbol in current TU or dependent files
Current algorithm tries to do the same as built-in
follow symbol but better.
Currently clang-based follow symbol has some limitations:
- following function usage may return the declaration
instead of definition because we don't have header
dependencies in backend
- overrides are not searched because of the same reason
and the amount of dependent files (parsing 250 files
takes a while)
- some includes are not handled correctly, in that case
we return failure and ask built-in code model to follow
(example: <QtGui> or other qt includes)

Change-Id: If35028ee0b5e818fdba29363c9520c5cca996348
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/tools/clangbackend/ipcsource/cursor.cpp

@@ -273,6 +273,11 @@ CXSourceRange Cursor::cxSourceRange() const
     return clang_getCursorExtent(cxCursor);
 }
 
+CXTranslationUnit Cursor::cxTranslationUnit() const
+{
+    return clang_Cursor_getTranslationUnit(cxCursor);
+}
+
 SourceRange Cursor::commentRange() const
 {
     return clang_Cursor_getCommentRange(cxCursor);