Fix the end location of init-capture annotations in ObjC++

And thereby stop asserting.

In ObjC++ modes, we tentatively parse the lambda introducer twice: once
to disambiguate designators, which we also do in C++, and a second time
to disambiguate objc message expressions. During the second tentative
parse, the last cached token will be the annotation token we built in
the first parse. So use getLastLoc() to get the correct end location
for the rebuilt annotation.

llvm-svn: 236246