[patch] libstdc++/59768 Fix std::invoke support for reference_wrappers

[Jonathan Wakely]

My attempt to implement LWG 2219 wasn't very good and resulted in
ambiguous overloads for the testcase in PR 59768. This fixes it, and
makes sure that const reference_wrappers can be used too.

Tested powerpc64le-linux, committed to trunk.


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