[NFC] Add a test case for missing 'try' diagnostics with existential opening.
[NFC] Add a test case for missing 'try' diagnostics with existential opening.

apple/swift,roambotics/swift,roambotics/swift,apple/swift,apple/swift,roambotics/swift,roambotics/swift,apple/swift,roambotics/swift,apple/swift,apple/swift,roambotics/swift