[libc++] Refactor vector::push_back to use vector::emplace (#113481)

This removes some duplicate code. I suspect this was originally written
that way because vector::emplace didn't exist in C++03 mode, which
stopped being relevant when Clang implemented rvalue references in
C++03.

NOKEYCHECK=True
GitOrigin-RevId: 8a7318eb00fcd833aefa8b0028690e9fae221ce6
1 file changed