[Sema] Adjust Sema::getCurBlock()/getCurLambda() to take into account that we may have
switch CurContext due to class template instantiation.

Fixes crash of the included test case.
rdar://16527205

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207325 91177308-0d34-0410-b5e6-96231b3b80d8