[clang] __is_trivially_equality_comparable for types containing lambdas (#68506)

Lambdas (closure types) are trivially equality-comparable iff they are
non-capturing, because non-capturing lambdas are convertible to function
pointers: if (lam1 == lam2) compiles, then lam1 and lam2 must have
the same type, and be always-equal, and be empty.