Rdar 28786959 3.0 branch if swift 3 digit version

[graydon]

Cherry-pick of PR against master: #5440

Initial PR description:

Two changes, close relatives.

First change enables writing #if swift(>=3.0.1) by contextually inhibiting an early diagnostic in the parser that's currently blocking the initial semi-parse that runs before the Version parser kicks in.

Second change weakens our sense of logical order and logical equality on Version structures such that 4 == 4.0 == 4.0.0 (that is, versions are considered to have as many trailing zeroes as necessary). The function evaluating versions you are allowed to pass on the command line is tightened opposite this change, so you still can't say -swift-version 4.0 (only -swift-version 4) but it means that if someone writes #if swift(>=4.0) it'll work, rather than the current behaviour, that thinks 4 < 4.0.

Resolves SR-2908.

[graydon]

@swift-ci please smoke test

[ematejska]

@swift-ci please test os x

[lattner]

Thanks @graydon !

[shahmishal]

@graydon Can you please check if this failure is related to this PR? Thanks!

https://ci.swift.org/view/swift-3.0-branch/job/oss-swift-3.0-incremental-RA-linux-ubuntu-14_04/315/consoleFull#-1080512551fca400bf-2f4a-462e-b517-e058d770b2d7

Script:
--
/home/buildnode/jenkins/workspace/oss-swift-3.0-incremental-RA-linux-ubuntu-14_04/buildbot_incremental/swift-linux-x86_64/bin/swift -frontend -target x86_64-unknown-linux-gnu  -parse -verify -disable-objc-attr-requires-foundation-module /home/buildnode/jenkins/workspace/oss-swift-3.0-incremental-RA-linux-ubuntu-14_04/swift/test/Parse/ConditionalCompilation/language_version.swift
--
Exit Code: 1

Command Output (stderr):
--
/home/buildnode/jenkins/workspace/oss-swift-3.0-incremental-RA-linux-ubuntu-14_04/swift/test/Parse/ConditionalCompilation/language_version.swift:55:3: error: unexpected error produced: use of unresolved identifier 'asdf'
  asdf asdf asdf asdf
  ^
/home/buildnode/jenkins/workspace/oss-swift-3.0-incremental-RA-linux-ubuntu-14_04/swift/test/Parse/ConditionalCompilation/language_version.swift:55:7: error: unexpected error produced: consecutive statements on a line must be separated by ';'
  asdf asdf asdf asdf
      ^
/home/buildnode/jenkins/workspace/oss-swift-3.0-incremental-RA-linux-ubuntu-14_04/swift/test/Parse/ConditionalCompilation/language_version.swift:55:8: error: unexpected error produced: use of unresolved identifier 'asdf'
  asdf asdf asdf asdf
       ^
/home/buildnode/jenkins/workspace/oss-swift-3.0-incremental-RA-linux-ubuntu-14_04/swift/test/Parse/ConditionalCompilation/language_version.swift:55:12: error: unexpected error produced: consecutive statements on a line must be separated by ';'
  asdf asdf asdf asdf
           ^
/home/buildnode/jenkins/workspace/oss-swift-3.0-incremental-RA-linux-ubuntu-14_04/swift/test/Parse/ConditionalCompilation/language_version.swift:55:13: error: unexpected error produced: use of unresolved identifier 'asdf'
  asdf asdf asdf asdf
            ^
/home/buildnode/jenkins/workspace/oss-swift-3.0-incremental-RA-linux-ubuntu-14_04/swift/test/Parse/ConditionalCompilation/language_version.swift:55:17: error: unexpected error produced: consecutive statements on a line must be separated by ';'
  asdf asdf asdf asdf
                ^
/home/buildnode/jenkins/workspace/oss-swift-3.0-incremental-RA-linux-ubuntu-14_04/swift/test/Parse/ConditionalCompilation/language_version.swift:55:18: error: unexpected error produced: use of unresolved identifier 'asdf'
  asdf asdf asdf asdf
                 ^

--

[graydon]

@shahmishal yes, that is caused by the linux host being set to Swift version: 3.0 rather than 3.0.1, which the test implicitly relies on. I apologize, I didn't realize linux wasn't calling itself 3.0.1 yet.

I've filed #5495 to fix. Not sure what the protocol is wrt. authorization to apply such a fix.