[libc++] P0355R7: `std::chrono::clock_time_conversion` and `std::chrono::clock_cast`

**Link:** https://wg21.link/p0355r7, https://eel.is/c++draft/time.clock.cast

Start by reading the linked P0355R7 proposal and the C++ draft section on time.clock.cast, then locate the existing libc++ chrono implementation and related conformance tests. The issue does not name files or tests, so first map the affected chrono entry points; done means the specified clock conversion and cast facilities are implemented with coverage for the standard wording.

Written by the indexing model from the issue text.