libstdc++: Fix std::runtime_format deviations from the spec [PR113320]

I seem to have implemented this feature based on the P2918R0 revision,
not the final P2918R2 one that was approved for C++26. This commit fixes
it.

The runtime-format-string type should not have a publicly accessible
data member, so add a constructor and make it a friend of
basic_format_string. It should also be non-copyable, so that it can only
be constructed from a prvalue via temporary materialization. Change the
basic_format_string constructor parameter to pass by value. Also add
noexcept to the constructors and runtime_format generator functions.

libstdc++-v3/ChangeLog:

	PR libstdc++/113320
	* include/std/format (__format::_Runtime_format_string): Add
	constructor and disable copy operations.
	(basic_format_string(_Runtime_format_string)): Add noexcept and
	take parameter by value not rvalue reference.
	(runtime_format): Add noexcept.
	* testsuite/std/format/runtime_format.cc: Check noexcept. Check
	that construction is only possible from prvalues, not xvalues.

Reviewed-by: Daniel Krügler <daniel.kruegler@gmail.com>

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