Use SHGetKnownFolderPath to get path to localappdata for current user

[stevenbrix]

On Windows, it's not guaranteed that USERPROFILE is equivalent to %SYSTEMDRIVE%\Users\%USERNAME%. So it's possible that UserDefaults tries to write to a non-existent location, in which case it will never work. When this happens, all writes silently no-op.

This PR uses the Shell API SHGetKnownFolderPath to get the location of %LOCALAPPDATA% instead of manually building the path.

See documentation for this API:

Fixes #4997

[ShikiSuen]

@stevenbrix You might want to remove the format changes introduced on your computer.
This minifies the modifications that only necessary changes are kept.

P.S.: This PR is really important and I don't know why it gets abandoned here for months.

[ShikiSuen]

@stevenbrix This PR is fallen behind the main branch too far.
The file to modified has been relocated to this folder:
/Sources/CoreFoundation/CFKnownLocations.c
And the part I guess you might want to modify is limited to this scope:

swift-corelibs-foundation/Sources/CoreFoundation/CFKnownLocations.c

Lines 97 to 123 in 0129358

	case _kCFKnownLocationUserCurrent:
	username = CFGetUserName();
	// fallthrough
	case _kCFKnownLocationUserByName: {
	DWORD size = 0;
	GetProfilesDirectoryW(NULL, &size);
	wchar_t* path = (wchar_t*)malloc(size * sizeof(wchar_t));
	GetProfilesDirectoryW(path, &size);
	CFStringRef pathRef = CFStringCreateWithCharacters(kCFAllocatorSystemDefault, path, size - 1);
	free(path);
	CFURLRef profilesDir = CFURLCreateWithFileSystemPath(kCFAllocatorSystemDefault, pathRef, kCFURLWindowsPathStyle, true);
	CFRelease(pathRef);
	CFURLRef usernameDir = CFURLCreateCopyAppendingPathComponent(kCFAllocatorSystemDefault, profilesDir, username, true);
	CFURLRef appdataDir = CFURLCreateCopyAppendingPathComponent(kCFAllocatorSystemDefault, usernameDir, CFSTR("AppData"), true);
	location = CFURLCreateCopyAppendingPathComponent(kCFAllocatorSystemDefault, appdataDir, CFSTR("Local"), true);
	CFRelease(usernameDir);
	CFRelease(appdataDir);
	CFRelease(profilesDir);
	if (user == _kCFKnownLocationUserCurrent) {
	CFRelease(username);
	}
	break;

[compnerd]

This seems incorrect. The path parameter is a wide C String not a CFStrimg. This should do the creation of the CFString and pass that.