fix: specify currency in locale identifier when formatting currency plural (#919)

Summary:
Intl.NumberFormat does not respect the currency input when currencyDisplay is "name". Without currencyDisplay, currency is correctly taken into account.

Resolves #914

Pull Request resolved: #919

Test Plan:
Build Hermes and installed on RN application, tested against multiple currency/locale pairs.

```
new Intl.NumberFormat('de-DE', {
  style: 'currency',
  currency,
  currencyDisplay: 'name',
}).format(2)
```

Reviewed By: mhorowitz

Differential Revision: D43437187

Pulled By: neildhar

fbshipit-source-id: 47a7b322fbb498e431b7db9130da3a852c5de873

Author: Asher Lim

lib/Platform/Intl/PlatformIntlApple.mm

@@ -2468,18 +2468,16 @@ uint8_t getCurrencyDigits(std::u16string_view code) {
   // - compactDisplay is not supported.
   // - signDisplay is not supported.
   // - NSNumberFormatter has maximumIntegerDigits, which is 42 by default
-  auto nsLocale =
-      [NSLocale localeWithLocaleIdentifier:u16StringToNSString(dataLocale_)];
+  std::u16string nsLocaleStr = dataLocale_;
   nsNumberFormatter_ = [NSNumberFormatter new];
-  nsNumberFormatter_.locale = nsLocale;
   if (style_ == u"decimal") {
     nsNumberFormatter_.numberStyle = NSNumberFormatterDecimalStyle;
     if (notation_ == u"scientific") {
       nsNumberFormatter_.numberStyle = NSNumberFormatterScientificStyle;
     }
   } else if (style_ == u"currency") {
     nsNumberFormatter_.numberStyle = NSNumberFormatterCurrencyStyle;
-    nsNumberFormatter_.currencyCode = u16StringToNSString(*currency_);
+    nsLocaleStr.append(u"@currency=").append(*currency_);
     if (currencyDisplay_ == u"code") {
       nsNumberFormatter_.numberStyle = NSNumberFormatterCurrencyISOCodeStyle;
     } else if (currencyDisplay_ == u"symbol") {
@@ -2507,6 +2505,9 @@ uint8_t getCurrencyDigits(std::u16string_view code) {
     nsNumberFormatter_.maximumSignificantDigits = significantDigits_->maximum;
   }
   nsNumberFormatter_.usesGroupingSeparator = useGrouping_;
+  auto nsLocale =
+      [NSLocale localeWithLocaleIdentifier:u16StringToNSString(nsLocaleStr)];
+  nsNumberFormatter_.locale = nsLocale;
   if (style_ == u"unit") {
     nsMeasurementFormatter_ = [NSMeasurementFormatter new];
     nsMeasurementFormatter_.numberFormatter = nsNumberFormatter_;