libstdc++: Enable memset optimizations for distinct character types [PR93059]

libstdc++: Enable memset optimizations for distinct character types [PR93059]

Currently we only optimize std::fill to memset when the source and
destination types are the same byte-sized type. This means that we fail
to optimize cases like std::fill(buf. buf+n, 0) because the literal 0 is
not the same type as the character buffer.

Such cases can safely be optimized to use memset, because assigning an
int (or other integer) to a narrow character type has the same effects
as converting the integer to unsigned char then copying it with memset.

This patch enables the optimized code path when the fill character is a
memcpy-able integer (using the new __memcpyable_integer trait). We still
need to check is_same<U, T> to enable the memset optimization for
filling a range of std::byte with a std::byte value, because that isn't
a memcpyable integer.

libstdc++-v3/ChangeLog:

	PR libstdc++/93059
	* include/bits/stl_algobase.h (__fill_a1(T*, T*, const T&)):
	Change template parameters and enable_if condition to allow the
	fill value to be an integer.