[Clang] Mark WG14 N2939 (Identifier Syntax Fixes) as available in Clang 15 (#74666)

WG14 N2939 (Identifier Syntax Fixes) corrects a grammar issue in the C
standard but does not otherwise change intended behavior. This change
updates the C23 status to note this paper as implemented as of Clang 15;
the release in which support for N2836 (Identifier Syntax using Unicode
Standard Annex 31) was implemented.