libstdc++: Fix name clash with _Cosh in QNX headers [PR 95592]

libstdc++: Fix name clash with _Cosh in QNX headers [PR 95592]

This replaces unqualified names like _Cosh with struct std::_Cosh to
ensure there is no ambiguity with other entities with the same name.

libstdc++-v3/ChangeLog:

	PR libstdc++/95592
	* include/bits/valarray_after.h (_DEFINE_EXPR_UNARY_OPERATOR)
	(_DEFINE_EXPR_BINARY_OPERATOR, _DEFINE_EXPR_BINARY_FUNCTION):
	Use elaborated-type-specifier and qualified-id to avoid
	ambiguities with QNX system headers.
	* testsuite/26_numerics/valarray/95592.cc: New test.