Add _runAsyncMain to api digester list

Adding `_runAsyncMain` to the api-digester list since we're adding
`@preconcurrency` when we add `@Sendable`. These two should negate each
other, but the api-digester isn't quite smart enough to understand that.

Author: etcwilde

test/api-digester/stability-concurrency-abi.test

@@ -58,6 +58,7 @@ Func AsyncSequence.map(_:) is now with @preconcurrency
 Func AsyncSequence.prefix(while:) is now with @preconcurrency
 Func MainActor.run(resultType:body:) has generic signature change from <T where T : Swift.Sendable> to <T>
 Func MainActor.run(resultType:body:) has mangled name changing from 'static Swift.MainActor.run<A where A: Swift.Sendable>(resultType: A.Type, body: @Swift.MainActor @Sendable () throws -> A) async throws -> A' to 'static Swift.MainActor.run<A>(resultType: A.Type, body: @Swift.MainActor @Sendable () throws -> A) async throws -> A'
+Func _runAsyncMain(_:) is now with @preconcurrency
 Protocol Actor has added inherited protocol AnyActor
 Protocol Actor has generic signature change from <Self : AnyObject, Self : Swift.Sendable> to <Self : _Concurrency.AnyActor>
 Struct CheckedContinuation has removed conformance to UnsafeSendable