STL: vNext should drop support for `/Zc:wchar_t-`

[StephanTLavavej]

The rationale for continuing to support /Zc:wchar_t- is binary compatibility. As vNext will break ABI, it seems reasonable to demand that wchar_t always be a real type.

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.

[StephanTLavavej]

Mentioned by Microsoft-internal VSO-387426.

[StephanTLavavej]

We talked about this at the weekly maintainer meeting and were thoroughly in agreement that we should drop this support if we can get away with it (i.e. if we can convince our internal partner teams that because vNext is inherently a recompile-the-world scenario, wchar_t should become a real type at that time; mismatched wchar_t settings are a persistent drain on productivity and vNext will be a chance to finally solve that problem.)