[ClangImporter] SE-0463: Implement `@Sendable` inference exception for global actor isolated functions
Functions that are isolated to a global actor don't have completion
handlers imported as `@Sendable`. Main actor isolated functions with
completion handlers that are always called on the main actor is a
very common Objective-C pattern, and this carve out will eliminate
false positive warnings in the cases where the main actor annotation
is missing on the completion handler parameter.
Resolves: rdar://149811049