[Sema] Add variable captured by a block to the enclosing lambda's

potential capture list.

Fix Sema::getCurLambda() to return the innermost lambda scope when there
is a block enclosed in the lambda. Previously, the method would return a
nullptr in such cases, which would prevent a variable captured by the
enclosed block to be added to the lambda scope's potential capture list.

rdar://problem/28412462

Differential Revision: https://reviews.llvm.org/D25556

llvm-svn: 296584