[PATCH] Add string_view support to COW std::string

[Jonathan Wakely]

This adds the C++17 changes to make std::string_view interoperate with
std::string. The code is mostly just copied from the new std::string,
with two adjustments needed where the names of internal member
functions differ.

With this we have no more test failures when using the old ABI (either
via running tests with -D_GLIBCXX_USE_CXX11_ABI=0 or when configured
so that's the default).

	PR libstdc++/79075
	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI] (basic_string):
	Make _If_sv private.
	[!_GLIBCXX_USE_CXX11_ABI] (basic_string): Add member functions taking
	basic_string_view arguments.

Tested powerpc64le-linux, committed to trunk.

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