[v3] libstdc++/16612

[Benjamin Kosnik <bkoz at redhat dot com>]

This patch removes the empty string::_Rep as a carefully constructed
cast of-externally-allocated-array idea in basic string and uses a
static default constructed string::_Rep instead.

This does allow the string class to be used with shared allocators. That
part of the patch is still pending copyright assignment, but will
hopefully be in mainline sources without too much of a wait. I'm
tracking the progress of this assignment and will put in the relevant
patch as soon as possible. For those who are curious, there is bugzilla.

There is a similar patch for libstdcxx_so_7 branch that removes the
backwards-compat cruft in this patch. The next time that branch is
merged, I'll throw this version of the patch in.

tested x86/linux
performance tested x86/linux

I'll wait 24 hrs on this for Nathan and Paolo to comment.

best,
benjamin

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