[5.7] SILGen: Function availability for linkage should use @_backDeploy OS version if present #61170
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Cherry-pick of #61169.
When computing the availability of a
SILFunctionfor linkage the OS version specified in the@_backDeployattribute should be preferred over the version in the@availableattribute. This ensures that the corresponding symbol is weakly linked when deploying to older OSes than the back deploy "before" version.Resolves rdar://99962885