Release notes: mention support for Unicode and UCNs in identifiers.

I'm using the name "Extended Identifiers" for the feature because that's
what GCC calls them. According to the standard, the new feature is
"universal character names are now allowed in identifiers", but the more
interesting "feature" is that identifiers can now contain Unicode characters,
however they are written.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174798 91177308-0d34-0410-b5e6-96231b3b80d8