[libc++] Mark LWG3211 as complete: default constructor of tuple<> should be trivial

`libc++` has had the guarantee of the default constructor of `tuple<>` being
trivial since 405570dc. Now, the
standard mandates it as of LWG3211. So, move the file out of
`libcxx/test/libcxx` and into `libcxx/test/std` since it's no longer
`libc++`-specific. Rename it to be `.compile.pass.cpp` instead of
`.pass.cpp` while we're at it.

Reviewed By: ldionne, Quuxplusone, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D112743