[patch] Use std::decay in return type of std::async (LWGW 2021)

[Jonathan Wakely]

http://cplusplus.github.io/LWG/lwg-defects.html#2021 corrects the
return type of std::async(), which is necessary to ensure that the
result_of expression matches the actual result of the callable object
passed to async.

We already do the right thing for std::bind(), so only std::async()
needs changes.

Tested powerpc64le-linux, committed to trunk.

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