Translate effective version name of '4.x' to '4.2' for ImportNameVersion.

[tkremenek]

The effective version for 4.x may be greater than '2', even with -swift-version 4.2. However, that should be canonicalized to 4.2.

[tkremenek]

@swift-ci please smoke test

[jrose-apple]

Strictly speaking I should ask you for a test… ;-) That can be a follow-up commit.

[tkremenek]

@jrose-apple A test is reasonable. What do you suggest? Currently the driver only accepts -swift-version 4.2, etc. The notion here was that the current tests should suffice when the effective version bumps up to something like (say) 4.3 under -swift-version 4.2 mode, if we decide to go that route. This is about future proofing on later version changes to the compiler.

If there is a good way to write a unit test here, happy to do so.

[jrose-apple]

You could write a gtest-style test instead, like unittests/Basic/SourceManagerTest.cpp.