[libc++] ECMAScript IdentityEscape is ambiguous (2584)

This patch fixes [[ https://cplusplus.github.io/LWG/issue2584 | 2584 ]]. Now the following works:
    const std::regex r1("\\z");
    assert(std::regex_match("z", r1));

Differential Revision: https://reviews.llvm.org/D66610