Combine instantiation context of field initializer with context of class.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236005 91177308-0d34-0410-b5e6-96231b3b80d8