[ADT] NFC: Fix bogus StringSwitch rule-of-five boilerplate

Now that 'Str' is constant, the rule-of-file logic needs updating.

Reported by: vit9696@avp.su
Reviewed by: jordan_rose@apple.com

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328803 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed