QAnyStringView should auto-detect ASCII in 8-bit constexpr inputs

Currently, as per Qt policy, const char* are treated as being encoded in UTF-8. But most such strings are actually US-ASCII, and therefore valid Latin-1. QAnyStringView, which can contain both, should scan the string at construction time for any 8th-bit character, and having found none, adjust the UTF-8 tag to Latin1. This should be a compile-time check only, no runtime code, so it naturally only applies when some form of string literal is being passed.

Acceptance criteria: The following tests (and others, with constexpr input) pass:

static_assert(QAnyStringView("Hello, World").isLatin1());
static_assert(QAnyStringView(u8"Hello, World").isLatin1());
static_assert(QAnyStringView::fromArray("Hello, World").isLatin1());
static_assert(QAnyStringView::fromArray(u8"Hello, World").isLatin1());

(isLatin1()) is private API atm; we could make the test a friend of QAnyStringView or use

any.visit([](auto view) { return std::is_same_v<decltype(view), QLatin1String>; })

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