Properly validate UCNs for C99 and C++03 (both more restrictive than C(++)11).

Add warnings under -Wc++11-compat, -Wc++98-compat, and -Wc99-compat when a
particular UCN is incompatible with a different standard, and -Wunicode when
a UCN refers to a surrogate character in C++03.

llvm-svn: 174788