<locale>: make collate::hash return the same hash for strings that collate the same

[muellerj2]

Fixes #5212.

At first glance, calling LCMapStringEx with LCMAP_HASH seems to be the obvious solution, but the docs state (emphasis mine):

Strings that appear equivalent typically return the same hash (for example, "hello" and "HELLO" with LCMAP_IGNORECASE). However, some complex cases, such as East Asian languages, can have similar strings with identical weights that compare as equal but do not return the same hash.

I briefly tested this and the warning seems to be true, as the following program finds some single-character strings that collate the same but yield different hashes:

#include <iostream>
#include <windows.h>

using namespace std;

int main() {
	const wchar_t* locale = L"ja-JP";
	for (wchar_t w = 1; w != 0; ++w) {
		int hashw;
		if (LCMapStringEx(locale, LCMAP_HASH, &w, 1, reinterpret_cast<LPWSTR>(&hashw), sizeof(int), nullptr, nullptr, 0) == 0) {
			continue;
		}

		for (wchar_t x = w + 1; x != 0; ++x) {
			int hashx;
			if (LCMapStringEx(locale, LCMAP_HASH, &x, 1, reinterpret_cast<LPWSTR>(&hashx), sizeof(int), nullptr, nullptr, 0) == 0) {
				continue;
			}
			if (hashw != hashx && CompareStringEx(locale, 0, &w, 1, &x, 1, nullptr, nullptr, 0) == CSTR_EQUAL) {
				cout << "found different hashes for characters collating the same at: " << int(w) << " : " << int(x) << '\n';
			}
		}
	}
}

Output:

found different hashes for characters collating the same at: 1556 : 64606
found different hashes for characters collating the same at: 1611 : 65137
found different hashes for characters collating the same at: 1614 : 65143
[...]

This means that LCMapStringEx with LCMAP_HASH is not good enough to meet the requirements in [locale.collate.virtuals]/3.

For this reason, this PR computes the sort key first (by calling do_transform) and then hashes the sort key.

[StephanTLavavej]

Thanks! 😻 I pushed a trivial nitpick and a change to guard this with SKIP_COLLATE_TRANSFORM_TESTS and a comment explaining why. Please meow if I got that wrong.