[CodeGenObjCXX] Fix handling of blocks in lambda.

This fixes a crash that occurs when a block captures a reference that is
captured by its enclosing lambda.

rdar://problem/18586651

Differential Revision: http://reviews.llvm.org/D19536


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268532 91177308-0d34-0410-b5e6-96231b3b80d8