[ThinLTO] Don't try to import alias unless aliasee can be imported

With r264503, aliases are now being added to the GlobalsToImport set
even when their aliasees can't be imported due to their linkage type.
While the importing worked correctly (the aliases imported as
declarations) due to the logic in doImportAsDefinition, there is no
point to adding them to the GlobalsToImport set.

Additionally, with D18487 it was resulting in incorrectly printing a
message indicating that the alias was imported.

To avoid this, delay adding aliases to the GlobalsToImport set until
after the linkage type of the aliasee is checked.

This patch is part of D18487.

llvm-svn: 264536