Fix value of platform_module_dir in lit.cfg

[gwynne]

In lit.cfg, the platform_module_dir variable's value is constructed based on the %target-sdk-name substitution. However, substitutions are not applied at this point in lit.cfg's logic, so the resulting value was always the actual string %target-sdk-name. Instead, use the actual value of target_sdk_name, making platform_module_dir correctly reflect the target SDK.

[jrose-apple]

I don't think this matters as long as it's only used in other substitutions. Did you want to use it somewhere else?

[gwynne]

@jrose-apple There were at least two cases where I saw the literal value actually used as the name of a directory on disk as part of the build - that was what originally brought my attention to the bug. It ultimately forms part of the value of %platform_sdk_overlay_dir, which is a substitution, but substitutions aren't recursive and one ends up with the literal string on disk unless a test_sdk_overlay_dir was set.

[jrose-apple]

Huh. That seems…very strange, since we have tests that use %platform_sdk_overlay_dir. But if you were able to cause a failure, then sure, let's take this. (Or reorder the substitutions.)

[gwynne]

The failure to make the replacement doesn't prevent anything from working that I could find, it just looks quite odd in the build output and filesystem. But since making it work properly doesn't break anything, my take is that it makes more sense to have it right in the event something does eventually depend on it.

[gwynne]

@swift-ci please smoke test