Combine instantiation context of field initializer with context of class.

Inclass initializer is instantiated in its own LocalInstantiationScope. It
causes problems when instantiating local classes - when instantiation scope
is searched for DeclContext of the field, the search fails. As a solution,
the instantiation scope of field initializer is combined with its outer
scope.

This patch fixes PR23194.

Differential Revision: http://reviews.llvm.org/D9258

llvm-svn: 236005