Improve diagnostics for UCNs referring to control characters and members of the

basic source character set in C++98. Add -Wc++98-compat diagnostics for same in
literals in C++11. Extend such support to cover string literals as well as
character literals, and mark N2170 as done.

This seems too minor to warrant a release note to me. Let me know if you disagree.

llvm-svn: 152444