[Concurrency] Fix an issue with Sendable checking for function references.

[hborla]

• Add a test case for passing a non-Sendable argument to a nonisolated closure in an actor-isolated context. This previously did not produce a diagnostic, but now does on main.
• Don't check parameter and result types of function references for Sendability. Sendability of function references depends only on captures, and applies when the function itself is passed across isolation boundaries. Parameter and result values can only cross isolation boundaries when the function is called, so they shouldn't be checked when checking a reference for Sendable violations.

Resolves rdar://104129168

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test Linux