Try to parse a declarator, if that fails an expression, if that fails,
...at last a TranslationUnit is tried. It is also possible to specify
which AST should be parsed.

This simplifies the code snippets we can pass to this tool.

Change-Id: Idbc1a8a6f1c5cf7e20d899f7a2e4263c7f9d33a6
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>