[Parse] Support #if swift(<4.1)

[hashemi]

Add the ability to specify conditional compilation using < in addition to the currently supported >= unary prefix. #if swift(<4.1) is equivalent to #if !swift(>=4.1).

Resolves #49401.

[xwu]

@swift-ci Please smoke test

[jrose-apple]

Thanks for doing this! Please include some tests that do not come out true, as well as at least one that uses !.

cc @rintaro for any additional comments

[jrose-apple]

@jckarter, @rjmccall, do you think this requires an evolution proposal as well? I tagged the original bug as StarterProposal, but it is a very small change.

[xwu]

To future-proof these tests, I think the conditions should be such that asdf asdf asdf asdf emits no diagnostics in an #if branch which is never executed--for example, #if swift(<4).

[rjmccall]

It's a language change. I think the Core Team could approve this without a review by acclamation, but it should at least be given the opportunity to formally say that.

[xwu]

This is a five-year time bomb. Why not Swift 99?

[rintaro]

Implementation itself looks good :)

[hashemi]

Reimplemented in #17960