Issue Description:

When obtaining a value for midnight using Calendar.startOfDay(for: Date) with a Time Zone set to Europe/Dublin, the value returned is for Asia/Kolkata.
It appears the issue stems from the fact that both time zones have the same abbreviation (IST).

The following code illustrates the issue:
======= BEGIN CODE
import Foundation

let now = Date()
print("Now: (now)")
var cal = Calendar.current

print("CAL: (cal), Locale: (cal.locale?.identifier), TZ: (cal.timeZone)")

let midnight = cal.startOfDay(for: now)
print("MID: (midnight)")
====== END CODE =======

To verify, set the time zone to Kolkata (timedatectl set-timezone Asia/Kolkata).
Execute the application.

Now set the timezone to Europe/London.
Execute the application.

Now set the timezone to Europe/Dublin.
Execute the application.

You will see that Europe/Dublin gives the same output as Asia/Kolkata.

timedatectl <ENTER> will verify that the timezone is set to Europe/Dublin, so the error must lie with the swift Calendar implementation.