At present, every async function is responsible for noticing it has
been passed a nullptr for async_info and initializing it. With this change,
a new api function is called to initialize async_info within libomptarget.
A subsequent change can then only call async functions when that construction
succeeded, at which point the init-if-passed-nullptr logic from the plugins
can be replaced with asserts.
Couple of typo fixes noticed while implementing. Builds on D96438.