C++: Fix crash for invalid code

...due to indirect recursion:

  ...
  CPlusPlus::ClassOrNamespace::lookupType        LookupContext.cpp 833  0x7fffd6c954cc
  CPlusPlus::ClassOrNamespace::nestedType        LookupContext.cpp 1364 0x7fffd6c94bc6
  CPlusPlus::ClassOrNamespace::lookupType_helper LookupContext.cpp 955  0x7fffd6c9517f
  CPlusPlus::ClassOrNamespace::lookupType_helper LookupContext.cpp 983  0x7fffd6c952ad
  CPlusPlus::ClassOrNamespace::lookupType        LookupContext.cpp 833  0x7fffd6c954cc
  CPlusPlus::ClassOrNamespace::nestedType        LookupContext.cpp 1364 0x7fffd6c94bc6
  CPlusPlus::ClassOrNamespace::lookupType_helper LookupContext.cpp 955  0x7fffd6c9517f
  CPlusPlus::ClassOrNamespace::lookupType_helper LookupContext.cpp 983  0x7fffd6c952ad
  CPlusPlus::ClassOrNamespace::lookupType        LookupContext.cpp 833  0x7fffd6c954cc
  ...

ClassOrNamespace::lookupType(const Name *) already guards with a list of
entries already processed, but some calls deeper the list is not passed
on and lookupType() starts again with an empty list. Handle that case,
too.

Task-number: QTCREATORBUG-18499
Change-Id: Iab8978f6ac1d0aea16f49b3547415f43de887b07
Reviewed-by: Marco Bubke <marco.bubke@qt.io>