[libc++] Make future_error constructor standard-compliant

This patch removes the non compliant constructor of std::future_error
and adds the standards compliant constructor in C++17 instead.

Note that we can't support the constructor as an extension in all
standard modes because it uses delegating constructors, which require
C++11. We could in theory support the constructor as an extension in
C++11 and C++14 only, however I believe it is acceptable not to do that
since I expect the breakage from this patch will be minimal.

If it turns out that more code than we expect is broken by this, we can
reconsider that decision.

This was found during D99515.

Differential Revision: https://reviews.llvm.org/D99567


Co-authored-by: Louis Dionne <ldionne.2@gmail.com>