Use the project-level swift version when not defined at the target-level

[benasher44]

Fixes #5737 and #5700

PodVariants are unique by, among other things, their TargetDefinition's swift version. Previously this would be nil if only defined at the project level. This would lead to multiple PodVariant instances for the same PodTarget incorrectly, and later on this leads to the Errno::EEXIST errors from incorrectly trying to create the same support files directories for the same PodTargets.

[endocrimes]

LGTM when CI Passes - Out of interest in what cases does Xcode generate these?

[benasher44]

If you set "Use Legacy Swift Language Version" to Yes or No in the Project then you get this.

[benasher44]

If by "generate" you mean the Errno::EEXIST errors, those are generated during pod install. The linked project in #5700 reproduces this reliably.

[benasher44]

The caveat here though is that if you only configure "Use Legacy Swift Language Version" in the project and not in the targets, then you won't run into any issues because the TargetDefinition#swift_version always returns nil, so technically all of the reported swift versions are the same 😃

[endocrimes]

@benasher44 makes sense

[benasher44]

Only failure is the spurious execution_output.txt one

[segiddins]