Patch for PR17934

[Marshall Clow]

http://llvm.org/bugs/show_bug.cgi?id=17934
	C++14 cbegin/cend crbegin/crend definitions don't see all overloads of begin/end rbegin/rend

Based the fix suggested by Peter Sommerlad.

Move some code around in <iterator> so that it's visible when the general templates get instantiated.
Add tests to make sure (for std::array).

It kind of bugs me that:
	std::initializer_list<int> il {1,2};
	std::rbegin(il);	// <-- this compiles
	il.rbegin();		// <-- this does not compile.

… but that's what the standard says.

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists at gmail.com>

A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait).
        -- Yu Suzuki
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