[Features] Rename the BuiltinBuildTaskExecutor feature guard.

[hborla]

The name of the TaskExecutor protocol was recently changed in #71145 to remove underscores after the task executor preference proposal was accepted in Swift Evolution. An implication of that rename is that the buildOrdinaryTaskExecutorRef builtin changed the type that it expected as the argument. However, the original _TaskExecutor protocol landed in the _Concurrency library which as since produced .swiftinterface files that contain the following @inlinable code:

@inlinable public init<E: _TaskExecutor>(ordinary executor: __shared E) {
  #if $BuiltinBuildTaskExecutor
  self.executor = Builtin.buildOrdinaryTaskExecutorRef(executor)
  #else
  fatalError("Swift compiler is incompatible with this SDK version")
  #endif
}

When a compiler containing the TaskExecutor protocol rename attempts to type check the above @inlinable code when rebuilding the _Concurrency module from its .swiftinterface file, it crashes because the builtin is expecting an argument conforming to TaskExecutor, which doesn't exist in this version of the standard library. Specifically, the crash happens here:

  case _taskExecutor:
    return SC.Context.getProtocol(KnownProtocolKind::TaskExecutor)
      ->getDeclaredInterfaceType();

The issue is that the current compiler still supports the $BuiltinBuildTaskExecutor feature guard, and thus attempts to typecheck self.executor = Builtin.buildOrdinaryTaskExecutorRef(executor), but the type of executor conforms to _TaskExecutor instead of TaskExecutor. Because TaskExecutor doesn't exist in this version of the _Concurrency library, SC.Context.getProtocol(KnownProtocolKind::TaskExecutor) returns nullptr.

To resolve this issue, we need to stop supporting the $BuiltinBuildTaskExecutor feature guard and introduce a new one that is only supported by compiler versions that contain the TaskExecutor rename. This approach relies on nothing having adopted the API, otherwise we would need to stage in the rename as a parallel set of APIs, and only remove the old APIs once nothing is relying on the old _Concurrency .swiftinterface files. With this rename of the feature guard, compilers type checking older _Concurrency.swiftinterface files will behave as if they do not support the feature, and thus will not attempt to type check the code guarded by #if $BuiltinBuildTaskExecutor.

[hborla]

@swift-ci please smoke test

[ktoso]

LGTM, thank you -- didn't realize we need to do that, will keep an eye out for those!