[libc++][NFC] Add a few explicit 'inline' keywords, mostly in <chrono> (#75234)

Even though constexpr implicitly makes functions inline, we try not to
rely on this implicit effect in the code base. We are mostly consistent
about using `inline` on non-template free-functions to make it clear
that we don't have an ODR violation.

This patch simply fixes a few places where we didn't explicitly use
inline on non-template free functions, presumably because they were
constexpr.

Fixes #75227