C++: add support for local types

This change addes support for class, enum definition inside blocks({}) or
functions, e.g.:
void f()
{
	struct S
	{
		int bar;
	};
	S s;
	s.bar;
}

It fixes:
* code completion
* highlighting
* follow symbol
* marking
* find usages

It fixes also problem with namespace aliases inside blocks or functions.

This change can have also impact on performance(there are additional processing)

Task-number: QTCREATORBUG-166 (namespace aliases inside function/block)
Task-number: QTCREATORBUG-3620
Task-number: QTCREATORBUG-6013
Task-number: QTCREATORBUG-8020
Change-Id: Iaea6c6dfe276f1d7b2279b50bdd2e68e375d31eb
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>