Defer building 'this' captures until we have left the capturing region

Defer building 'this' captures until we have left the capturing region
and returned to the context in which 'this' should be captured.

This means we now always mark 'this' referenced from the context in
which it's actually referenced, rather than potentially from some
context nested within that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@362182 91177308-0d34-0410-b5e6-96231b3b80d8