[RuntimeAsync] ConfigureAwait(bool) is not optimized for ValueTask-returning method.

[VSadov]

Observed while enabling rt async in Libraires partition - #119432

While the optimization should not be needed for correctness, thus not the first priority to fix, it is certainly not the intention to leave this unoptimized.

The reason why we do not recognize the await pattern in Await(call).ConfigureAwait(false) when call returns ValueTask is because we expect ConfigureAwait applied to the result, but the IL contains stloc;ldloca in between.

The importer (impMatchTaskAwaitPattern) needs to learn about this.

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