Fixed a Visual Studio 2017 build error. (#4488)

[rcane]

The current 15.6.x versions of Visual Studio 2017 contain a bug that
prevent them from compiling the following construct under certain
conditions:

std::unique_ptr<std::unique_ptr []> foos;

This will fail to compile if Foo is an abstract class. To work-around
the problem the whole construct was change into:

std::vector<std::unique_ptr> foos;

This not only fixes the compiler error but is also more readable than
previous version.

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[rcane]

I signed it!

[googlebot]

CLAs look good, thanks!

[acozzette]

ok to test (this comment will trigger some other CI runs)

[xfxyjwf]

@acozzette Jenkins has been turned down already so we no longer need to reply "ok to test" to trigger those tests. The kokoro tests use a different trigger: the "kokoro:run" label. You just need to apply the label and kokro will pick it up (and remove the label so you can lable it again to trigger another run if needed).