If a clang module declares a cross-import overlay, but it also has a traditional overlay, we want the cross-import overlay to be registered with the SourceFile as sitting atop the traditional overlay. Otherwise module-qualified name lookups will bypass the cross-import overlay.
Fixes rdar://62139656.
Temporarily replace with `SourceFile::setImports`
until import resolution is requestified. Now
imports are only set once for a given SourceFile.
Because we're now asserting in more places that
import resolution must have run before querying
imports, this commit also adds
`getCachedUnderlyingType` to TypeAliasDecl to stop
the ASTDumper from trying to query imports for a
-dump-parse invocation.
Add ModuleImplicitImportsRequest, which computes
the modules that should be implicitly imported by
each file of a given module. Use this request in
import resolution to add all the necessary
implicit imports.
The request computes the implicit imports by
consulting the ImplicitImportInfo, which ModuleDecl
can now be created with. This allows us to remove
uses of `SourceFile::addImports` in favor of
adding modules needed to be implicitly imported to
the ImplicitImportInfo.
