This warnings don't give much benefits for developers. Code completion
UI tends to show them unusable. But usually, developers can modify the
context to accept async calls, e.g. by wrapping it with `Task { }`
rdar://126737530
* Implement 'getDiagnosticSeverity()' and 'getDiagnosticMessage()' on
'CodeCompletionResult'
* Differentiate 'RedundantImportIndirect' from 'RedundantImport'
* Make non-Sendable check respects '-warn-concurrency'
rdar://76129658