[libc++][NFC] rename __is_trivially_equality_comparable to...

[libc++][NFC] rename __is_trivially_equality_comparable to __libcpp_is_trivially_equality_comparable

This is required for D147175.

Reviewed By: ldionne, Mordante, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D147953