[libc++] Enable [[nodiscard]] extensions by default

[libc++] Enable [[nodiscard]] extensions by default

Adding `[[nodiscard]]` to functions is a conforming extension and done extensively in the MSVC STL.

Reviewed By: ldionne, EricWF, #libc

Spies: #libc_vendors, cjdb, mgrang, jloser, libcxx-commits

Differential Revision: https://reviews.llvm.org/D128267