Extend the _unsafeInheritExecutor_ workaround to all remaining APIs in the
Concurrency library that have adopted `#isolation` default arguments to
(safely) stay in the caller's isolation domain...
... except one. Clock.measure() is currently running into problems with
the type checker workaround and needs a little more thought.
Fixes rdar://131760111.
While the new parameter is added in a compatible way where code which
does not refer to it will get a defaulted nil value; since we refer to a
new parameter name in source, we need to guard it with a language
feature -- as old compilers will not have this new name available.
This should prevent a potential condfail issue.
