[Clang] Add support for Unicode identifiers (UAX31) in C2x mode.

This implements
N2836 Identifier Syntax using Unicode Standard Annex 31.

The feature was already implemented for C++,
and the semantics are the same.

Unlike C++ there was, afaict, no decision to
backport the feature in older languages mode,
so C17 and earlier are not modified and the
code point tables for these language modes are conserved.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D130416