don't assert when a module re-exports a module and one of its symbols

[QuietMisdreavus]

Resolves rdar://106807038

SymbolGraphGen currently asserts if a module includes re-exports like the following:

@_exported import OtherModule
@_exported import class OtherModule.SomeClass

This is because there's a validity check in the code that handles re-exports to ensure that duplicate symbol data does not end up in the symbol graph. However, this is valid Swift and should be treated as such.

This PR updates the exported-import handler to skip qualified re-exports that come from a module that is also being re-exported as a whole. The qualified symbol in question should already be counted because it is in the module being blanket exported.

[QuietMisdreavus]

@swift-ci Please smoke test

[QuietMisdreavus]

I double-checked the symbol graph generated by Swift 5.7.2 (in Xcode 14.2) in macOS where assertions were disabled, and it seems like the "duplicate" symbols were already being filtered out by SymbolGraphGen's use of a DenseSet to store symbols. It may be worthwhile to remove this assertion entirely, especially since it would remove the extra iteration of the symbol listing and creation of a new set in assert builds.

[artemcm]

This seems like a reasonable change to me.