Discrepancy: `String.data(using: .japaneseEUC)` returns `nil` on non-Darwin.

[YOCKOW]

Given the following code:

import Foundation
print("Hello, world!".data(using: .japaneseEUC) as Any)

Results are:

OS	Swift	Result
macOS	5.10	"Optional(13 bytes)"
Ubuntu	5.10	"nil"
macOS	6.0	"Optional(13 bytes)"
Ubuntu	6.0	"nil"

This is not regression but discrepancy which should be also fixed.

[parkera]

Resolved by #1217 as long as Foundation is imported.

[parkera]

Actually - @YOCKOW can you verify that?

[YOCKOW]

While I confirmed ISO-2022-JP was OK, EUC-JP seems still broken.

Edit: I'm sorry. This issue is not regression. It has been existing from a long time ago. I'll try to tackle on this separately...