We were using std::basic_string_view as a stand-in for std::span so that
we could change over all at once when C++20 dropped with full span
support. That day's not here yet, but as of 54a7fce3e we're using GSL 3,
whose span is C++20-compliant.
This commit replaces every instance of basic_string_view that was not
referring to an actual string with a span of the appropriate type.
I moved the `const` qualifier into span's `T` because while
`basic_string_view.at()` returns `const T&`, `span.at()` returns `T&`
(without the const). I wanted to maintain the invariant that members of
the span were immutable.
* Mechanical Changes
* `sv.at(x)` -> `gsl::at(sp, x)`
* `sv.c{begin,end}` -> `sp.{begin,end}` (span's iterators are const)
I had to replace a `std::basic_string<>` with a `std::vector<>` in
ConImeInfo, and I chose to replace a manual array walk in
ScreenInfoUiaProviderBase with a ranged-for. Please review those
specifically.
This will almost certainly cause a code size regression in Windows
because I'm blowing out all the PGO counts. Whoops.
Related: #3956, #975.
