CplusPlus: Fix minimal name construction

... when using declarations are involved.
Second attempt.
I have a suspicion that this is not the "correct" approach, however:
    a) It appears to fix the problem.
    b) It does not break any tests.
    c) It's rather high in the call tree, so there's a low likelihood
       of breaking something else.
    d) Considering the hacky-looking pre-existing code dealing with
       using declarations (e.g. LookupContext::lookupByUsing()),
       it seems quite possible that there is no "correct" way with
       the current infrastructure.

Fixes: QTCREATORBUG-14524
Change-Id: I28c6e53137c21ce158bbd03b92b3db39146116d5
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/cplusplus/LookupContext.h

@@ -83,6 +83,7 @@ public:
     ClassOrNamespace *lookupType(const Name *name, Block *block);
     ClassOrNamespace *findType(const Name *name);
     ClassOrNamespace *findBlock(Block *block);
+    ClassOrNamespace *getNested(const Name *name);
 
     Symbol *lookupInScope(const QList<const Name *> &fullName);