constexpr basic_string_view as required by C++17

[Benjamin Buch]

The current implementation does miss a lot of constexpr in
basic_string_view. (Bug 70483)

The patch adds the missing constexpr's as required by C++17. I did run
the the stdlibc++-v3 testsuite and got no errors. I didn't changes the
basic_string_view test cases, because I couldn't find a pattern in it
how to test if the function declarations are constexpr. (This is my
first patch for GCC.)
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