This commit adds a fix-it to remove @discardableResult on functions that return Void or Never. The fix-it is at the warning level. A test was added to verify that the fix-it removes the @discardableResult. This issue was reported in SR-3359:
https://bugs.swift.org/browse/SR-3359
Changes:
TypeCheckAttr.cpp: implemented AttributeChecker::visitDiscardableResultAttr to add a fix-it to remove @discardableResult on functions returning Void or Never.
DiagnosticsSema.def: Added a warning with a diagnostic message.
LoggingWrappers.swift.gyb, HashedCollections.swift.gyb: Removed @discardableResult on functions returning Void.
fixits-apply-all.swift, fixits-apply-all.swift.result: Added tests to verify that @discardableResult is removed from functions returning Void or Never.