PR18560: When switching to a new context, don't just save and restore an

override for the type of 'this', also clear it out (unless we're entering the
context of a lambda-expression, where it should be inherited).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199962 91177308-0d34-0410-b5e6-96231b3b80d8