Remove -Wc++98-compat warning for an outrageously-rare circumstance of 'this'

being used in an exception specification in a way which isn't otherwise
ill-formed in C++98: this warning also incorrectly triggered on uses of 'this'
inside thread-safety attributes, and the mechanism required to tell these cases
apart is more complex than can be justified by the (minimal) value of this part
of -Wc++98-compat.

llvm-svn: 155857