[patch] libstdc++/56383 Fix ambiguity with multiple enable_shared_from_this bases

[Jonathan Wakely]

The friend function defined in enable_shared_from_this did not match
the declaration at namespace scope, so instead of defining the
previously declared function it added a new overload, which was always
a better match than the declared (but never defined) one.

That worked fine for a single base class, the friend function got
found by ADL, but with two enable_shared_from_this base classes the
two friend overloads were ambiguous.

This changes the friend to match the declaration, so that it can only
be called when an unambiguous enable_shared_from_this base can be
deduced, and so fails silently (as it is supposed to) when there is
not an unambiguous base class.

Tested powerpc64le-linux, committed to trunk.

This fix is simple enough that I'm going to backport it after 5.3 is
released.

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