libstdc++: Replace use of reserved name that clashes [PR 97362]

libstdc++: Replace use of reserved name that clashes [PR 97362]

The name __deref is defined as a macro by Windows headers.

This renames the __deref() helper function to __ref. It doesn't actually
dereference an iterator. it just has the same type as the iterator's
reference type.

libstdc++-v3/ChangeLog:

	PR libstdc++/97362
	* doc/html/manual/source_code_style.html: Regenerate.
	* doc/xml/manual/appendix_contributing.xml: Add __deref to
	BADNAMES.
	* include/debug/functions.h (_Irreflexive_checker::__deref):
	Rename to __ref.
	* testsuite/17_intro/badnames.cc: Check __deref.