[libc++] ADL-proof <thread>, and eliminate `using namespace chrono`.

Since we know exactly which identifiers we expect to find in `chrono`,
a using-directive seems like massive overkill. Remove the directives
and qualify the names as needed.

One subtle trick here: In two places I replaced `*__p` with `*__p.get()`.
The former is an unqualified call to `operator*` on a class type, which
triggers ADL and breaks the new test. The latter is a call to the
built-in `operator*` on pointers, which specifically
does NOT trigger ADL thanks to [over.match.oper]/1.

Differential Revision: https://reviews.llvm.org/D92243