This commit changes fixit messages from a question/suggestion to an
imperative message for protocol conformances and switch-case. Addresses
https://github.com/apple/swift/issues/67510.
If the setter conflict occurs in a deserialized declaration, the parent
pattern binding can be NULL. Guard the fixit on the existence of the
pattern binding so
1) we don't crash
2) we don't try to emit a fixit in otherwise extremely broken code
rdar://56558082
