The call to __valarray_copy constructs an _Array object to refer to
this->_M_data but that means that accesses to this->_M_data are through
a restrict-qualified pointer. This leads to undefined behaviour when
copying from an _Expr object that actually aliases this->_M_data.

Replace the call to __valarray_copy with a plain loop. I think this
removes the only use of that overload of __valarray_copy, so it could
probably be removed. I haven't done that here.

libstdc++-v3/ChangeLog:

	PR libstdc++/99117
	* include/std/valarray (valarray::operator=(const _Expr&)):
	Use loop to copy instead of __valarray_copy with _Array.
	* testsuite/26_numerics/valarray/99117.cc: New test.