P2255R2: A type trait to detect reference binding to temporary

[ldionne]

Link: https://wg21.link/P2255R2

[H-G-Hristov]

It looks like the required builtins have been implemented:

__reference_constructs_from_temporary added in https://reviews.llvm.org/D135341 / 8f3b0b4

__reference_converts_from_temporary added in #91199 / 2dbe89d

Issue: #114344

__has_builtin() issues fixed in 75611ca