[libc++] Fix lifetime issues of temporaries.

The ASAN build failed due to using pointers to a temporary whose
lifetime had expired.

Updating the libc++ Docker image to Ubuntu Focal caused some breakage.
This was temporary disabled in D112737. This re-enables two of these
tests.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D113137