[libc++] Remove workarounds for missing rvalue references

We don't support GCC in C++03 mode, and Clang provides rvalue references
even in C++03 mode. So there's effectively no supported compiler that
doesn't support rvalue references.

Differential Revision: https://reviews.llvm.org/D84943

GitOrigin-RevId: 8d4860aa9ee78f4c948b5f38a043c527d49d6a34
18 files changed