[SR-3168] Add fix-it for "'optional' can only be applied to protocol members"

[robinkunde]

This PR adds a fixit for cases in which the keyword "optional" is incorrectly applied to class or struct methods.

Resolves SR-3168.

[robinkunde]

@swift-ci Please smoke test

[rintaro]

@robinkunde Thanks!
test/FixCode/fixits-apply.swift is not the primary place for testing diagnostics.
Could you update test/decl/protocol/req/optional.swift instead?

[robinkunde]

@rintaro Maybe I'm misunderstanding, but I didn't touch the diagnostic itself and it's already covered in test/decl/protocol/req/optional.swift starting at line 207. Do you want me to set up that test file to test fixits as well?

[rintaro]

Yeah, testing fix-its can be done with:

  optional var ivar: Int // expected-error{{'optional' can only be applied to protocol members}} {{3-14=}}

The format is:

{{3-14=something}}
   \ \     `- replacing text
    \ `- end column
     `- start column

[robinkunde]

@rintaro Thanks for pointing that out! I've amended the commit.

[rintaro]

@swift-ci Please smoke test

[rintaro]

@swift-ci Please smoke test

[rintaro]

Merging. Thank you @robinkunde !