<xlocale>: codecvt_base::result should be an enum type

[StephanTLavavej]

STL/stl/inc/xlocale

Lines 664 to 670 in 58c5c4d

	enum { // constants for different parse states
	ok,
	partial,
	error,
	noconv
	};
	using result = int;

https://eel.is/c++draft/locale.codecvt mandates enum result { ok, partial, error, noconv };.

(Standardese citations as of WG21-N4835.)

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.