libstdc++: Make std::assume_aligned a constexpr function [PR 97132]

libstdc++: Make std::assume_aligned a constexpr function [PR 97132]

The cast from void* to T* in std::assume_aligned is not valid in a
constexpr function. The optimization hint is redundant during constant
evaluation anyway (the compiler can see the object and knows its
alignment). Simply return the original pointer without applying the
__builtin_assume_aligned hint to it when doing constant evaluation.

libstdc++-v3/ChangeLog:

	PR libstdc++/97132
	* include/std/memory (assume_aligned): Do not use
	__builtin_assume_aligned during constant evaluation.
	* testsuite/20_util/assume_aligned/1.cc: Improve test.
	* testsuite/20_util/assume_aligned/97132.cc: New test.

(cherry picked from commit f10ed928e2f8ecc2c859abff8f2f9296b11b8d95)