<hash_set|hash_map>: Add opt-out to the stdext namespace

[tiagomacarios]

Perhaps a feature request (or a conformance bug) - Would it be possible to make the stdext disappear behind a control macro?
_HAS_STDEXT_NAMESPACE = 1 -> current behavior
_HAS_STDEXT_NAMESPACE = 0 -> stdext is removed by the preprocessor

This would enable large codebases from prevent users from using this features. It would also enable us to eventually default to 0 and have users need to opt-in until this is completely removed.

[StephanTLavavej]

<hash_map> and <hash_set> are specifically blocked with a hard #error until the user defines a macro, so it's impossible to use them accidentally:

STL/stl/inc/hash_map

Lines 19 to 22 in 0408152

	#ifndef _SILENCE_STDEXT_HASH_DEPRECATION_WARNINGS
	#error <hash_map> is deprecated and will be REMOVED. Please use <unordered_map>. You can define \
	_SILENCE_STDEXT_HASH_DEPRECATION_WARNINGS to suppress this error.
	#endif // _SILENCE_STDEXT_HASH_DEPRECATION_WARNINGS

For the other stdext namespace stuff, which we've been working on deprecating, adding a mode to control its existence (like tr1) entirely seems reasonable.

[frederick-vs-ja]

In <exception>, it seems that the stdext emulates inline namespace and allows TUs with exception turned on/off to be linked together.