<string_view>: suppress warning about annotation 'NullTerminated'

[fsb4000]

Fixes warning C6510: Invalid annotation: 'NullTerminated' property may only be used on buffers whose elements are of integral or pointer type: Function '{ctor}' _Param_(1).

https://gcc.godbolt.org/z/bPqs167he

Can we do something else and don't remove the annotation but fix the warning?
Found in one of LLVM tests.

[frederick-vs-ja]

Perhaps we can use requires to separate this ctor into two overloads, one of which only accepts raw pointers and is annotated with _In_z.
(I don't want to templatize this ctor, but it seems that templatization is a feasible way when requires is absent.)

[CaseyCarter]

Just a few extraneous std::-qualifications, I'll apply changes.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for nulling out this warning! ⚠️ ✅ 😸