libstdc++: Simplify std::launder definition

A single static assert is a much simpler way to implement the
compile-time preconditions on std::launder than an overload set of
deleted functions and function templates. The only difficulty is that
<new> doesn't include <type_traits> so we can't use std::is_function and
std::is_void for the checks. That can be worked around though, by using
the __is_same and __is_function built-ins. If the __is_function built-in
isn't supported then the __builtin_launder built-in will give an error
anyway, since the commit preceding this one.

We can also remove the redundant __cplusplus >= 201703L check around the
definitions of std::launder and the interference constants, which are
already guarded by the appropriate feature test macros.

libstdc++-v3/ChangeLog:

	* libsupc++/new (launder): Add static_assert and remove deleted
	overloads.
	* testsuite/18_support/launder/requirements_neg.cc: Adjust
	expected diagnostics.

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