libstdc++: Fix some warnings in headers

libstdc++: Fix some warnings in headers

These are usually suppressed in system headers, but should be fixed
anyway.

libstdc++-v3/ChangeLog:

	* include/bits/parse_numbers.h (_Select_int_base): Avoid
	narrowing conversion in constant expression.
	* include/experimental/buffer (buffer_copy): Avoid narrowing
	conversion.
	* include/experimental/internet (hash<>::operator()): Do not
	use deprecated 'argument_type' member.
	* include/std/variant (variant::emplace): Use cast instead
	of implicit conversion from size_t to narrower unsigned type.