Merging r312892:

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r312892 | ericwf | 2017-09-10 16:41:20 -0700 (Sun, 10 Sep 2017) | 10 lines

Fix PR34298 - Allow std::function with an incomplete return type.

This patch fixes llvm.org/PR34298. Previously libc++ incorrectly evaluated
the __invokable trait via the converting constructor `function(Tp)` [with Tp = std::function]
whenever the copy constructor or copy assignment operator
was required. This patch further constrains that constructor to short
circut before evaluating the troublesome SFINAE when `Tp` matches
std::function.

The original patch is from Alex Lorenz.
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llvm-svn: 318835