Implement C++20 constexpr <algorithm>, , and <array>

["Ed Smith-Rowland via libstdc++" <libstdc++ at gcc dot gnu dot org>]

Greetings,

This patch implements C++20 p0202 - Add Constexpr Modifiers to Functions 
in <algorithm> and <utility> Headers

and C++20 p1023 - constexpr comparison operators for std::array.


The patch is large because of the testsuite additions. Basically, the 
algorithms and the array comparison operators are all marked constexpr 
for C++20.  This has been built and tested on x86_64-linux.

OK?

Ed

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